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 4 minutes and 28 seconds
Status Job ID Name Coverage
  Docker Image
passed #4260066
debian-testing

00:00:59

 
  Build And Test
passed #4260067
autotools

00:03:28

passed #4260068
meson

00:01:23

passed #4260069
meson-noglamor

00:01:03