HID: multitouch: Add NULL check in mt_input_configured

devm_kasprintf() can return a NULL pointer on failure,but this
returned value in mt_input_configured() is not checked.
Add NULL check in mt_input_configured(), to handle kernel NULL
pointer dereference error.

Fixes: 47943946 ("HID: multitouch: Correct devm device reference for hidinput input_dev name")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>