resolve: initialize `r` during OOM

resolve: initialize `r` during OOM

Otherwise we'd use some garbage value in the error path.

../src/resolve/resolved-dns-query.c: In function ‘dns_query_accept’:
../src/resolve/resolved-dns-query.c:944:27: error: ‘r’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  944 |         q->answer_errno = -r;
      |                           ^~
cc1: all warnings being treated as errors

Follow-up for 9ca133e97a0.