Fix XRecordRegisterClients() Integer underflow

CVE-2020-14362 ZDI-CAN-11574

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

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
4 jobs for master in 6 minutes and 53 seconds
Status Job ID Name Coverage
  Docker Image
passed #4259905
debian-testing

00:00:29

 
  Build And Test
passed #4259906
autotools

00:06:23

passed #4259907
meson

00:02:03

passed #4259908
meson-noglamor

00:02:00