Fix unbalanced lock when serializing CDR variables

I'm only surprised that this didn't cause larger problems.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@395089 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/cdr.c

@@ -3054,7 +3054,7 @@ int ast_cdr_serialize_variables(const char *channel_name, struct ast_str **buf,
 			total++;
 		}
 	}
-
+	ao2_unlock(cdr);
 	return total;
 }