isdn/gigaset: fix NULL-deref at probe

commit 68c32f9c2a36d410aa242e661506e5b2c2764179 upstream.

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer should a malicious device lack endpoints.

Fixes: cf7776dc ("[PATCH] isdn4linux: Siemens Gigaset drivers - direct USB connection")
Cc: Hansjoerg Lipp <hjlipp@web.de>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Corey Edwards <ensabahnur16@gmail.com>