gpiolib: cdev: fix NULL-pointer dereferences

In the Linux kernel, the following vulnerability has been resolved:

gpiolib: cdev: fix NULL-pointer dereferences

There are several places where we can crash the kernel by requesting

lines, unbinding the GPIO device, then calling any of the system calls

relevant to the GPIO character device's annonymous file descriptors:

ioctl(), read(), poll().

While I observed it with the GPIO simulator, it will also happen for any

of the GPIO devices that can be hot-unplugged - for instance any HID GPIO

expander (e.g. CP2112).

This affects both v1 and v2 uAPI.

This fixes it partially by checking if gdev->chip is not NULL but it

doesn't entirely remedy the situation as we still have a race condition

in which another thread can remove the device after the check.