xfree86: Avoid crash in xf86RandR12CrtcSetGamma() memcpy path.
If randrp->palette_size is zero, the memcpy() path can read past the end of the randr_crtc's gammaRed/Green/Blue tables if the hw crtc's gamma_size is greater than the randr_crtc's gammaSize. Avoid this by clamping the to-be-copied size to the smaller of both sizes. Note that during regular server startup, the memcpy() path is only taken initially twice, but then a suitable palette is created for use during a session. Therefore during an actual running X-Session, the xf86RandR12CrtcComputeGamma() will be used, which makes sure that data is properly up- or down-sampled for mismatching source and target crtc gamma sizes. This should avoid reading past randr_crtc gamma memory for gpu's with big crtc->gamma_size, e.g., AMD/MALI/KOMEDA 4096 slots, or Intel Icelake and later with 262145 slots. Tested against modesetting-ddx and amdgpu-ddx under screen color depth 24 (8 bpc) and 30 (10 bpc) to make sure that clamping happens properly. This is an alternative fix for the one attempted in commit 617f591f . Signed-off-by: Mario Kleiner <email@example.com>