selftests/hid: add a test case for the recent syzbot underflow

Syzbot found a buffer underflow in __hid_request(). Add a related test
case for it.

It's not perfect, but it allows to catch a corner case when a report
descriptor is crafted so that it has a size of 0.

Link: https://patch.msgid.link/20250710-report-size-null-v2-4-ccf922b7c4e5@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>