res_resolver_unbound: Test for NULL ub_result in unbound_resolver_callback

res_resolver_unbound: Test for NULL ub_result in unbound_resolver_callback

The ub_result pointer passed to unbound_resolver_callback by
libunbound can be NULL if the query was for something malformed
like `.1` or `[.1]`.  If it is, we now set a 'ns_r_formerr' result
and return instead of crashing with a SEGV.  This causes pjproject
to simply cancel the transaction with a "No answer record in the DNS
response" error.  The existing "off nominal" unit test was also
updated to check this condition.

Although not necessary for this fix, we also made
ast_dns_resolver_completed() tolerant of a NULL result.

Resolves: GHSA-v428-g3cw-7hv9