Remove redundant code in dns.c

Peter J Philipp pointed out that there are two checks that ensure that len is
not less than 0. If len is less than 0, the function returns. Having both of
them is clearly redundant.

This removes the second and attempts to clarify (slightly) the error condition.

(closes issue ASTERISK-21772)
Reported by: Peter J Philipp


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394305 65c4cc65-6c06-0410-ace0-fbb531ad65f3