Currently, Xwayland pixmap SHM code uses `malloc()` to allocate the
xwl_pixmap.
Use `calloc()` instead, as the EGLstream backend does, as it is safer
(initializing the allocated data to 0).
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>