Mailbox: Check valid registered callback before calling

This patch checks if the mailbox user has assinged a valid
callback fuction before calling it.
Signed-off-by: Fernando Guzman Lugo <x0095840@ti.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>

arch/arm/plat-omap/mailbox.c

@@ -146,7 +146,8 @@ static void mbox_rx_work(struct work_struct *work)
 
 		msg = (mbox_msg_t)rq->special;
 		blk_end_request_all(rq, 0);
-		mbox->rxq->callback((void *)msg);
+		if (mbox->rxq->callback)
+			mbox->rxq->callback((void *)msg);
 	}
 }