The vb2_core_qbuf() function didn't check if q->error was set. It is
checked in __buf_prepare(), but that function isn't called if the buffer
was already prepared before with VIDIOC_PREPARE_BUF.

So check it at the start of vb2_core_qbuf() as well.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Acked-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>