Fix buffer overflow in parsing ipv6 ptr queries

[mu-b]

Fixes a buffer overflow (or should) in parsing ipv6 ptr queries.

In normal circumstances (buildbot builds) you are saved by the compiler with the inlining padding the stack and nothing useful to overwrite for now.

Building without optimisation and debugging:

root@OpenWrt:~# gdb /usr/sbin/umdns
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Reading symbols from /usr/sbin/umdns...
(gdb) r -i br-lan
Starting program: /usr/sbin/umdns -i br-lan
No IP address specified for unicast interface

Program received signal SIGILL, Illegal instruction.
0xb6fa86e8 in ?? ()
(gdb) bt
#0  0xb6fa86e8 in ?? ()
#1  0x000166fc in match_ipv6_addresses (
    reverse_ip=0x22108 <name_buffer> "AAAAAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA.ABBBAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA.AAAAAAAA", intf_ip=0xb6f1da98) at /home/aaaa/build/openwrt/build_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/umdns-2025.10.04~2f75344f/dns.c:468
#2  0x4242423a in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)

https://digit-labs.org/files/exploits/owrt-umdns.c