fix the return value of gettag() to only return the pointer to the provided

fix the return value of gettag() to only return the pointer to the provided
tag buffer if the tag was actually found.  There is code that checks to see
if this result is non-zero to determine whether the tag was found or not.
(issue #7092, mikma)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@25060 65c4cc65-6c06-0410-ace0-fbb531ad65f3