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)