Input: hanwang - 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: bba5394a

 ("Input: add support for Hanwang tablets")
Signed-off-by: Johan Hovold <johan@kernel.org>
Cc: stable@vger.kernel.org	# 2.6.37
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>