dns_core: Allow zero-length DNS responses.

A testsuite test recently failed due to a crash that occurred in the DNS
core. The problem was that the test could not resolve an address, did
not set a result on the DNS query, and then indicated the query was
completed. The DNS core does not handle the case of a query with no
result gracefully, and so there is a crash.

This changeset makes the DNS system resolver set a result with a
zero-length answer in the case that a DNS resolution failure occurs
early. The DNS core now also will accept such a response without
treating it as invalid input. A unit test was updated to no longer treat
setting a zero-length response as off-nominal.

Change-Id: Ie56641e22debdaa61459e1c9a042e23b78affbf6

main/dns.c

@@ -559,6 +559,7 @@ enum ast_dns_search_result ast_search_dns_ex(void *context, const char *dname, i
 
 	if (dns_response_len < 0) {
 		ast_log(LOG_ERROR, "DNS search failed for %s\n", dname);
+		response_handler(context, (unsigned char *)"", 0, ns_r_nxdomain);
 		return AST_DNS_SEARCH_FAILURE;
 	}