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

include/asterisk/dns_resolver.h

@@ -90,6 +90,10 @@ void *ast_dns_resolver_get_data(const struct ast_dns_query *query);
  * \param answer The raw DNS answer
  * \param answer_size The size of the raw DNS answer
  *
+ * Zero-sized and NULL answers are permitted by this function. This may be
+ * necessary if the query fails at an early stage and no actual DNS response
+ * has been received from a DNS server.
+ *
  * \retval 0 success
  * \retval -1 failure
  */