xkb: add request length validation for XkbSetGeometry

No validation of the various fields on that report were done, so a
malicious client could send a short request that claims it had N
sections, or rows, or keys, and the server would process the request for
N sections, running out of bounds of the actual request data.

Fix this by adding size checks to ensure our data is valid.

ZDI-CAN 16062, CVE-2022-2319.

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>