[media] usbvision: fix NULL-deref at probe

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer or accessing memory beyond the endpoint array should a
malicious device lack the expected endpoints.

Fixes: 2a9f8b5d ("V4L/DVB (5206): Usbvision: set alternate interface
modification")

Cc: stable <stable@vger.kernel.org>     # 2.6.21
Cc: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>