[PATCH for v5.3] videobuf-core.c: poll_wait needs a non-NULL buf pointer

[Hans Verkuil]

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-ci...@xs4all.nl>
Reported-by: Dan Carpenter <dan.carpen...@oracle.com>
Fixes: bb436cbeb918 ("media: videobuf: fix epoll() by calling poll_wait first")
Cc: <sta...@vger.kernel.org>      # for v5.1 and up
---
diff --git a/drivers/media/v4l2-core/videobuf-core.c 
b/drivers/media/v4l2-core/videobuf-core.c
index 7ef3e4d22bf6..939fc11cf080 100644
--- a/drivers/media/v4l2-core/videobuf-core.c
+++ b/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) {