dns_responder: Return an error if failed to build a resource record

If failed to build an expected RDATA resource record, the DNS
responder should not add the empty resource record into the answer
section of DNS response message.

Test: built, flashed, booted
      system/netd/tests/runtests.sh passes

Change-Id: I0d786f05142e9065c53266b9be79b86423b6919d