usb: gadget: configfs: Fix OOB read on empty string write

usb: gadget: configfs: Fix OOB read on empty string write

When writing an empty string to either 'qw_sign' or 'landingPage'
sysfs attributes, the store functions attempt to access page[l - 1]
before validating that the length 'l' is greater than zero.

This patch fixes the vulnerability by adding a check at the beginning
of os_desc_qw_sign_store() and webusb_landingPage_store() to handle
the zero-length input case gracefully by returning immediately.

Signed-off-by: Xinyu Liu <katieeliu@tencent.com>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/tencent_B1C9481688D0E95E7362AB2E999DE8048207@qq.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>