xkb: reset the radio_groups pointer to NULL after freeing it

Unlike other elements of the keymap, this pointer was freed but not
reset. On a subsequent XkbGetKbdByName request, the server may access
already freed memory.

CVE-2022-4283, ZDI-CAN-19530

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>