CLEANUP: resolvers: remove pool_free(NULL) in SRV additional record matching

CLEANUP: resolvers: remove pool_free(NULL) in SRV additional record matching

In resolv_validate_dns_response(), when matching an additional A/AAAA
record to an SRV record, the code checked tmp_record->ar_item == NULL
then called pool_free(resolv_answer_item_pool, tmp_record->ar_item).
This is a copy-paste mistake from similar patterns elsewhere since
the pointer is confirmed to be NULL a few lines above, so let's just
drop the confusing pool_free.