isdn/gigaset: fix NULL-deref at probe

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: stable <stable@vger.kernel.org>	# 2.6.17
Cc: Hansjoerg Lipp <hjlipp@web.de>

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>