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>
4 jobs for master in 4 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #3896684
debian-testing

00:00:18

 
  Build And Test
passed #3896685
autotools

00:04:17

passed #3896686
meson

00:02:22

passed #3896687
meson-noglamor

00:01:00