media: Quit parsing stream if doesn't start with SOI

In the case we get an invalid stream, such as from v4l2-compliance
streaming test, jpeg_next_marker will end up parsing the entire
stream. The standard describes the high level syntax of a jpeg
as starting with SOI, ending with EOI, so return error if the very
first 2 bytes are not SOI.

Signed-off-by: Mirela Rabulea <mirela.rabulea@nxp.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>