Message.c: Clear message channel frames on cleanup

The message channel is a special channel that doesn't actually process frames.
However, certain actions can cause frames to be placed in the channel's read
queue including the Hangup application which is called on the channel after
each message is processed. Since the channel will continually be reused for
many messages, it's necessary to flush these frames at some point.

ASTERISK-25083 #close
Reported by: Jonathan Rose

Change-Id: Idf18df73ccd8c220be38743335b5c79c2a4c0d0f

main/message.c

@@ -743,6 +743,7 @@ static void chan_cleanup(struct ast_channel *chan)
 	struct ast_datastore *msg_ds, *ds;
 	struct varshead *headp;
 	struct ast_var_t *vardata;
+	struct ast_frame *cur;
 
 	ast_channel_lock(chan);
 
@@ -771,6 +772,13 @@ static void chan_cleanup(struct ast_channel *chan)
 		ast_var_delete(vardata);
 	}
 
+	/*
+	 * Remove frames from read queue
+	 */
+	while ((cur = AST_LIST_REMOVE_HEAD(ast_channel_readq(chan), frame_list))) {
+		ast_frfree(cur);
+	}
+
 	/*
 	 * Restore msg datastore.
 	 */