Otherwise sys_pitch will be stale when a system memory copy is allocated.
Fixes https://bugs.freedesktop.org/show_bug.cgi?id=38322 and a crash when
unlocking the screen with xscreensaver, reported by Janne Huttunen.
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Janne Huttunen <jahuttun@gmail.com>
Tested-by: Jan Kriho <Erbureth@gmail.com>
Signed-off-by: Keith Packard <keithp@keithp.com>