Fix XChangeFeedbackControl() request underflow

CVE-2021-3472 / ZDI-CAN-1259

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

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
3 jobs for server-1.20-branch in 4 minutes and 56 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Docker Image
passed debian-testing #8821525

00:00:30

 
  Build And Test
passed autotools-build-and-test #8821526

00:04:26

passed meson-build-and-test #8821527

00:01:13