zd1211rw: 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: a1030e92 ("[PATCH] zd1211rw: Convert installer CDROM device into WLAN device")
Cc: Daniel Drake <dsd@gentoo.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>