astobj2: assert on invalid ref and backtrace cleanup

If a reference count goes negative, instead of
just logging that fact, be more helpful with a
backtrace and an assert that will DO_CRASH.

This patch also removes the duplicate ao2_bt()
function and cleans up extraneous usage of the
ast_log_backtrace() call.

Review: https://reviewboard.asterisk.org/r/3765/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@418963 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/astobj2.h

@@ -1961,9 +1961,6 @@ void *__ao2_iterator_next(struct ao2_iterator *iter) attribute_warn_unused_resul
  */
 void ao2_iterator_restart(struct ao2_iterator *iter);
 
-/* extra functions */
-void ao2_bt(void);	/* backtrace */
-
 /*! gcc __attribute__(cleanup()) functions
  * \note they must be able to handle NULL parameters because most of the
  * allocation/find functions can fail and we don't want to try to tear