HID: core: Harden s32ton() against conversion to 0 bits

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

HID: core: Harden s32ton() against conversion to 0 bits

Testing by the syzbot fuzzer showed that the HID core gets a shift-out-of-bounds exception when it tries to convert a 32-bit quantity to a 0-bit quantity. Ideally this should never occur, but there are buggy devices and some might have a report field with size set to zero; we shouldn't reject the report or the device just because of that.

Instead, harden the s32ton() routine so that it returns a reasonable result instead of crashing when it is called with the number of bits set to 0 -- the same as what snto32() does.