media: videobuf2-core: check for q->error in vb2_core_qbuf()

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: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>