add a return NULL to a function that is expected to return a value

add a return NULL to a function that is expected to return a value
so compilers that don't understand that this code is NOTREACHED
will not complain (the fault is not much on the compiler but on the
declaration of pthread_exit on certain platforms)
s/certain platform/cygwin/ if you are really curious

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89368 65c4cc65-6c06-0410-ace0-fbb531ad65f3