If rcode is SERVFAIL, there is no need to look at the packet.

If rcode is SERVFAIL, there is no need to look at the packet.

This pulls the check for rcode up, before we check if the answer
packet has sensible length. Since we are not touching the packet at
all, we don't care about the size and don't need to log if the size is
wrong from a DNS perspective.

With asr error reporting improved in the previous commit, this
probably gets rid of all "bad packet: too short" messages.

OK semarie