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>