If attempting to free a NULL pointer when MALLOC_DEBUG

is set, don't bother searching for a region to free, just
immediately exit.

This has the dual benefit of suppressing a warning message
about freeing memory at (nil) and of optimizing the free()
replacement by not having to do any futile searching for
the proper region to free.

(closes issue #13498)
Reported by: pj
Patches:
      13498.patch uploaded by putnopvut (license 60)
Tested by: pj



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@143400 65c4cc65-6c06-0410-ace0-fbb531ad65f3