go7007: add sanity checking for endpoints

A malicious USB device may lack endpoints the driver assumes to exist
Accessing them leads to NULL pointer accesses. This patch introduces
sanity checking.

Reported-and-tested-by:  <syzbot+cabfa4b5b05ff6be4ef0@syzkaller.appspotmail.com>

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Fixes: 866b8695 ("Staging: add the go7007 video driver")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>