xfree86/modes: Bail from xf86RotateRedisplay if pScreen->root is NULL

Merged Michel Dänzer requested to merge daenzer/xserver:xf86RotateRedisplay-no-root-bail into master

Avoids a crash in xf86RotatePrepare -> DamageRegister during CreateScreenResources if rotation or another transform is configured for any connected RandR output in xorg.conf. The generic rotation/transform code generally can't work without the root window currently.

Closes: #969 (closed) Fixes: 094f42cd "xfree86/modes: Call xf86RotateRedisplay from xf86CrtcRotate"

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