BUILD: resolvers: avoid a possible warning on null-deref

BUILD: resolvers: avoid a possible warning on null-deref

Depending on the code that precedes the loop, gcc may emit this warning:

  src/resolvers.c: In function 'resolv_process_responses':
  src/resolvers.c:1009:11: warning: potential null pointer dereference [-Wnull-dereference]
   1009 |  if (query->type != DNS_RTYPE_SRV && flags & DNS_FLAG_TRUNCATED) {
        |      ~~~~~^~~~~~

However after carefully checking, r_res->header.qdcount it exclusively 1
when reaching this place, which forces the for() loop to enter for at
least one iteration, and <query> to be set. Thus there's no code path
leading to a null deref. It's possibly just because the assignment is
too far and the compiler cannot figure that the condition is always OK.
Let's just mark it to please the compiler.