modesetting: Fix ms_covering_crtc() segfault with non-xf86Crtc slave

DIX ScreenRecs don't necessarily have an xf86CrtcConfigPtr DDX private.
ms_covering_crtc() assumes that they do, which can result in a segfault.

Update ms_covering_crtc() to check the XF86_CRTC_CONFIG_PTR() returned pointer
before dereferencing it. This will still mean that ms_covering_crtc() can't fall
back to the primary output when a drawable overlaps a slave output (going to the
1 FPS default instead), but it won't segfault.

Signed-off-by: Alex Goins <agoins@nvidia.com>
(cherry picked from commit 797e7a0ceb)

hw/xfree86/drivers/modesetting/vblank.c

@@ -141,6 +141,10 @@ ms_covering_crtc(ScreenPtr pScreen, BoxPtr box, Bool screen_is_ms)
 
     best_crtc = NULL;
     best_coverage = 0;
+
+    if (!xf86_config)
+        return NULL;
+
     for (c = 0; c < xf86_config->num_crtc; c++) {
         crtc = xf86_config->crtc[c];