Fix for segfault in __ast_rwlock_destroy with DEBUG_THREADS

If DEBUG_THREADS is enabled __ast_rwlock_destroy causes a segfault while trying
to access a possible NULL t->track object.  A NULL check has been added before
trying to access the memory.

(closes issue ASTERISK-21724)
Reported by: Corey Farrell
Fixed by: Corey Farrell
Patches:
	ast_rwlock_destroy-segv.patch uploaded by Corey Farrell (license 5909)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@388838 f38db490-d61c-443f-a65b-d21fe96a405b