media: videobuf-core.c: poll_wait needs a non-NULL buf pointer

poll_wait uses &buf->done, but buf is NULL. Move the poll_wait to later
in the function once buf is correctly set and only call it if it is
non-NULL.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: bb436cbe ("media: videobuf: fix epoll() by calling poll_wait first")
Cc: <stable@vger.kernel.org>      # for v5.1 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/v4l2-core/videobuf-core.c

@@ -1123,7 +1123,6 @@ __poll_t videobuf_poll_stream(struct file *file,
 	struct videobuf_buffer *buf = NULL;
 	__poll_t rc = 0;
 
-	poll_wait(file, &buf->done, wait);
 	videobuf_queue_lock(q);
 	if (q->streaming) {
 		if (!list_empty(&q->stream))
@@ -1143,7 +1142,9 @@ __poll_t videobuf_poll_stream(struct file *file,
 		}
 		buf = q->read_buf;
 	}
-	if (!buf)
+	if (buf)
+		poll_wait(file, &buf->done, wait);
+	else
 		rc = EPOLLERR;
 
 	if (0 == rc) {