astobj2: Correct ao2_iterator opacity violations

This corrects the ao2_iterator opacity violations in
res_pjsip_session.c by adding a global function to get the number of
elements inside the container hidden behind the iterator.

(closes issue ASTERISK-23053)
Review: https://reviewboard.asterisk.org/r/3111/
Reported by: Richard Mudgett
........

Merged revisions 405253 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405254 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/astobj2.h

@@ -1933,4 +1933,13 @@ void __ao2_cleanup_debug(void *obj, const char *file, int line, const char *func
 #endif
 void ao2_iterator_cleanup(struct ao2_iterator *iter);
 
+/*!
+ * \brief Get a count of the iterated container objects.
+ *
+ * \param iter the iterator to query
+ *
+ * \retval The number of objects in the iterated container
+ */
+int ao2_iterator_count(struct ao2_iterator *iter);
+
 #endif /* _ASTERISK_ASTOBJ2_H */