fix for ZDI-11426

Avoid leaking un-initalized memory to clients by zeroing the
whole pixmap on initial allocation.

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

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
(cherry picked from commit a6b2cbe9)
3 jobs for server-1.20-branch in 5 minutes and 9 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #4129683
debian-testing

00:00:50

 
  Build And Test
passed #4129684
autotools-build-and-test

00:04:19

passed #4129685
meson-build-and-test

00:01:28