Xi: avoid integer truncation in length check of ProcXIChangeProperty

This fixes an OOB read and the resulting information disclosure.

Length calculation for the request was clipped to a 32-bit integer. With
the correct stuff->num_items value the expected request size was
truncated, passing the REQUEST_FIXED_SIZE check.

The server then proceeded with reading at least stuff->num_items bytes
(depending on stuff->format) from the request and stuffing whatever it
finds into the property. In the process it would also allocate at least
stuff->num_items bytes, i.e. 4GB.

The same bug exists in ProcChangeProperty and ProcXChangeDeviceProperty,
so let's fix that too.

CVE-2022-46344, ZDI-CAN 19405

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>