Staging: hv: vmbus: Check before invoking the channel callback

When we close a channel, we set the corresponding callback function to NULL.
Check before invoking the channel callback.
Signed-off-by: NK. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: NHaiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/hv/connection.c

@@ -222,7 +222,7 @@ static void process_chn_event(u32 relid)
 	 */
 	channel = relid2channel(relid);
 
-	if (channel) {
+	if (channel && (channel->onchannel_callback != NULL)) {
 		channel->onchannel_callback(channel->channel_callback_context);
 	} else {
 		pr_err("channel not found for relid - %u\n", relid);