Make sure the devPrivate.ptr of a textured pixmap is NULL.

Just in case when wrongly fallback to DDX layer and cause
random memory corruption. Pointed out by Chris.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>

glamor/glamor.c

@@ -116,6 +116,7 @@ glamor_set_pixmap_texture(PixmapPtr pixmap, int w, int h, unsigned int tex)
 					      pixmap->drawable.
 					      bitsPerPixel + 7) / 8) +
 					    3) & ~3, NULL);
+	pixmap->devPrivate.ptr = NULL;
 }
 
 /* Set screen pixmap. If tex equal to 0, means it is called from ephyr. */