Potential memory leak in `wallet_clean_utxos`

[cdecker]

The following potential error was detected by CI (could be a false positive due to a pending RPC call being interrupted by the memleak call, in which case we need to ensure we keep a pointer):

_________________ ERROR at teardown of test_txprepare_restart __________________
[gw23] linux -- Python 3.6.8 /usr/bin/python3

request = <SubRequest 'teardown_checks' for <Function test_txprepare_restart>>

    @pytest.fixture
    def teardown_checks(request):
        """A simple fixture to collect errors during teardown.

        We need to collect the errors and raise them as the very last step in the
        fixture tree, otherwise some fixtures may not be cleaned up
        correctly. Require this fixture in all other fixtures that need to either
        cleanup before reporting an error or want to add an error that is to be
        reported.

        """
        errors = TeardownErrors()
        yield errors

        if errors.has_errors():
            # Format a nice list of everything that went wrong and raise an exception
            request.node.has_errors = True
>           raise ValueError(str(errors))
E           ValueError:
E           Node errors:
E           Global errors:
E            - Node /tmp/ltests-25ah4teu/test_txprepare_restart_1/lightning-1/ has memory leaks: [
E               {
E                   "backtrace": [
E                       "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E                       "wallet/wallet.c:154 (wallet_stmt2output)",
E                       "wallet/wallet.c:275 (wallet_get_utxos)",
E                       "wallet/wallet.c:3751 (wallet_clean_utxos)",
E                       "lightningd/lightningd.c:914 (main)"
E                   ],
E                   "label": "wallet/wallet.c:154:struct utxo",
E                   "parents": [
E                       "wallet/wallet.c:273:struct utxo*[]"
E                   ],
E                   "value": "0x564672eca9d8"
E               },
E               {
E                   "backtrace": [
E                       "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E                       "wallet/wallet.c:154 (wallet_stmt2output)",
E                       "wallet/wallet.c:275 (wallet_get_utxos)",
E                       "wallet/wallet.c:3751 (wallet_clean_utxos)",
E                       "lightningd/lightningd.c:914 (main)"
E                   ],
E                   "label": "wallet/wallet.c:154:struct utxo",
E                   "parents": [
E                       "wallet/wallet.c:273:struct utxo*[]"
E                   ],
E                   "value": "0x564672ecdb78"
E               },
E               {
E                   "backtrace": [
E                       "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E                       "wallet/wallet.c:154 (wallet_stmt2output)",
E                       "wallet/wallet.c:275 (wallet_get_utxos)",
E                       "wallet/wallet.c:3751 (wallet_clean_utxos)",
E                       "lightningd/lightningd.c:914 (main)"
E                   ],
E                   "label": "wallet/wallet.c:154:struct utxo",
E                   "parents": [
E                       "wallet/wallet.c:273:struct utxo*[]"
E                   ],
E                   "value": "0x564672ec0158"
E               },
E               {
E                   "backtrace": [
E                       "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E                       "wallet/wallet.c:154 (wallet_stmt2output)",
E                       "wallet/wallet.c:275 (wallet_get_utxos)",
E                       "wallet/wallet.c:3751 (wallet_clean_utxos)",
E                       "lightningd/lightningd.c:914 (main)"
E                   ],
E                   "label": "wallet/wallet.c:154:struct utxo",
E                   "parents": [
E                       "wallet/wallet.c:273:struct utxo*[]"
E                   ],
E                   "value": "0x564672ebdbb8"
E               },
E               {
E                   "backtrace": [
E                       "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E                       "wallet/wallet.c:154 (wallet_stmt2output)",
E                       "wallet/wallet.c:275 (wallet_get_utxos)",
E                       "wallet/wallet.c:3751 (wallet_clean_utxos)",
E                       "lightningd/lightningd.c:914 (main)"
E                   ],
E                   "label": "wallet/wallet.c:154:struct utxo",
E                   "parents": [
E                       "wallet/wallet.c:273:struct utxo*[]"
E                   ],
E                   "value": "0x564672eecbf8"
E               },
E               {
E                   "backtrace": [
E                       "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E                       "wallet/wallet.c:154 (wallet_stmt2output)",
E                       "wallet/wallet.c:275 (wallet_get_utxos)",
E                       "wallet/wallet.c:3751 (wallet_clean_utxos)",
E                       "lightningd/lightningd.c:914 (main)"
E                   ],
E                   "label": "wallet/wallet.c:154:struct utxo",
E                   "parents": [
E                       "wallet/wallet.c:273:struct utxo*[]"
E                   ],
E                   "value": "0x564672eca288"
E               }
E           ]

contrib/pyln-testing/pyln/testing/fixtures.py:171: ValueError

[cdecker]

Since this is our on-startup cleanup it'll most likely be fixed by #3789 or replacements.

[cdecker]

Fixed by #3825