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>
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