Ensure REF_DEBUG records entrys for attempts to ao2_ref an invalid object

This change ensures that __ao2_ref_debug writes to ref_log when given a
non-NULL pointer to an invalid ao2 object.  This is to ensure that we
record any attempt manipulate references of already freed objects.

ASTERISK-23948 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3677/
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Merged revisions 417500 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 417505 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 417509 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417511 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/astobj2.c

@@ -499,14 +499,8 @@ int __ao2_ref_debug(void *user_data, int delta, const char *tag, const char *fil
 {
 	struct astobj2 *obj = INTERNAL_OBJ(user_data);
 
-	if (obj == NULL) {
-		ast_log_backtrace();
-		ast_assert(0);
-		return -1;
-	}
-
-	if (ref_log) {
-		if (obj->priv_data.ref_counter + delta == 0) {
+	if (ref_log && user_data) {
+		if (obj && obj->priv_data.ref_counter + delta == 0) {
 			fprintf(ref_log, "%p,%d,%d,%s,%d,%s,**destructor**,%s\n", user_data, delta, ast_get_tid(), file, line, func, tag);
 			fflush(ref_log);
 		} else if (delta != 0) {
@@ -515,6 +509,13 @@ int __ao2_ref_debug(void *user_data, int delta, const char *tag, const char *fil
 			fflush(ref_log);
 		}
 	}
+
+	if (obj == NULL) {
+		ast_log_backtrace();
+		ast_assert(0);
+		return -1;
+	}
+
 	return internal_ao2_ref(user_data, delta, file, line, func);
 }