interface: fix possible null pointer dereference

interface: fix possible null pointer dereference

Fixes following issue reported by clang-12 static analyzer:

 mdnsd/interface.c:250:6: warning: Access to field 'ipi_ifindex' results in a dereference of a null pointer (loaded from variable 'inp')
         if (inp->ipi_ifindex != iface->ifindex)
             ^~~~~~~~~~~~~~~~

 mdnsd/interface.c:323:6: warning: Access to field 'ipi6_ifindex' results in a dereference of a null pointer (loaded from variable 'inp')
         if (inp->ipi6_ifindex != iface->ifindex)
             ^~~~~~~~~~~~~~~~~

Signed-off-by: Petr Štetiar <ynezz@true.cz>