fix warning and crash

[LubinLew]

Warning

build on ubuntu 24.04(x64) Server.

src/iface.cc: In static member function ‘static int ndppd::iface::poll_all()’:
src/iface.cc:667:90: warning: ignoring return value of ‘const _CharT* std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::c_str() const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’, declared with attribute ‘nodiscard’ [-Wunused-result]
  667 |                 logger::error() << "Failed to read from interface '%s'", ifa->_name.c_str();
      |                                                                          ~~~~~~~~~~~~~~~~^~
In file included from /usr/include/c++/13/string:54,
                 from src/iface.cc:38:
/usr/include/c++/13/bits/basic_string.h:2595:7: note: declared here
 2595 |       c_str() const _GLIBCXX_NOEXCEPT
      |       ^~~~~
src/iface.cc:706:90: warning: ignoring return value of ‘const _CharT* std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::c_str() const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’, declared with attribute ‘nodiscard’ [-Wunused-result]
  706 |                 logger::error() << "Failed to read from interface '%s'", ifa->_name.c_str();
      |                                                                          ~~~~~~~~~~~~~~~~^~
/usr/include/c++/13/bits/basic_string.h:2595:7: note: declared here
 2595 |       c_str() const _GLIBCXX_NOEXCEPT
      |       ^~~~~
g++ -c  -O3 -o src/proxy.o src/proxy.cc
g++ -c  -O3 -o src/address.o src/address.cc
g++ -c  -O3 -o src/rule.o src/rule.cc
g++ -c  -O3 -o src/session.o src/session.cc
src/session.cc: In member function ‘void ndppd::session::handle_auto_wire(const ndppd::address&, const std::string&, bool)’:
src/session.cc:212:15: warning: ignoring return value of ‘int system(const char*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  212 |         system(route_cmd.str().c_str());
      |         ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
src/session.cc:236:15: warning: ignoring return value of ‘int system(const char*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  236 |         system(route_cmd.str().c_str());
      |         ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
src/session.cc: In member function ‘void ndppd::session::handle_auto_unwire(const std::string&)’:
src/session.cc:264:15: warning: ignoring return value of ‘int system(const char*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  264 |         system(route_cmd.str().c_str());
      |         ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
src/session.cc:280:15: warning: ignoring return value of ‘int system(const char*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  280 |         system(route_cmd.str().c_str());
      |         ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

Error

When rule is static，ru->daughter() may be nullptr.

ndppd/src/iface.cc

Line 568 in 4f0301f

ru->daughter()->name() == ifname)