Input: yealink - validate number of endpoints before using them

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

Fixes: aca951a2 ("[PATCH] input-driver-yealink-P1K-usb-phone")
Signed-off-by: Johan Hovold <johan@kernel.org>
Cc: stable@vger.kernel.org	# 2.6.14
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>