BUG/MINOR: resolvers: classify empty truncated responses

The answer count is checked before the response question is parsed.
Most DNS servers return a truncated response without answers, so such
a response is classified as ANCOUNT_ZERO before the TC flag and
question type are examined. It increments the wrong counter and hides
the actual response status.

Move the empty-answer check after question parsing and TC handling.
Truncated SRV responses keep their existing partial-answer behavior.

This should be backported as far as 2.4.