record: Fix OOB access in ProcRecordUnregisterClients

If a client sends a RecordUnregisterClients request with an nClients
field larger than INT_MAX / 4, an integer overflow leads to an
out of boundary access in RecordSanityCheckClientSpecifiers.

An example line with libXtst would be:
XRecordUnregisterClients(dpy, rc, clients, 0x40000001);

Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 40c12a76)