[media] media: mem2mem: eliminate possible NULL pointer dereference

This patch removes the possible NULL pointer dereference in mem2mem
code.

Signed-off-by: Andrzej Pietrasiewicz <[e-mail]>
Signed-off-by: Kyungmin Park <[e-mail]>
Signed-off-by: Marek Szyprowski <[e-mail]>
CC: Pawel Osciak <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/video/v4l2-mem2mem.c

···
 
 	spin_lock_irqsave(&q_ctx->rdy_spinlock, flags);
 
-	if (list_empty(&q_ctx->rdy_queue))
-		goto end;
+	if (list_empty(&q_ctx->rdy_queue)) {
+		spin_unlock_irqrestore(&q_ctx->rdy_spinlock, flags);
+		return NULL;
+	}
 
 	b = list_entry(q_ctx->rdy_queue.next, struct v4l2_m2m_buffer, list);
-end:
 	spin_unlock_irqrestore(&q_ctx->rdy_spinlock, flags);
 	return &b->vb;
 }
···
 	unsigned long flags;
 
 	spin_lock_irqsave(&q_ctx->rdy_spinlock, flags);
-	if (!list_empty(&q_ctx->rdy_queue)) {
-		b = list_entry(q_ctx->rdy_queue.next, struct v4l2_m2m_buffer,
-				list);
-		list_del(&b->list);
-		q_ctx->num_rdy--;
+	if (list_empty(&q_ctx->rdy_queue)) {
+		spin_unlock_irqrestore(&q_ctx->rdy_spinlock, flags);
+		return NULL;
 	}
+	b = list_entry(q_ctx->rdy_queue.next, struct v4l2_m2m_buffer, list);
+	list_del(&b->list);
+	q_ctx->num_rdy--;
 	spin_unlock_irqrestore(&q_ctx->rdy_spinlock, flags);
 
 	return &b->vb;