[PR #2192] fb: quiet -Wanalyzer-out-of-bounds warnings in fbOverlayCopyWindow()

PR: https://github.com/X11Libre/xserver/pull/2192

fb/fboverlay.c

@@ -24,6 +24,7 @@
 
 #include <dix-config.h>
 
+#include <assert.h>
 #include <stdlib.h>
 
 #include "include/shmint.h"
@@ -210,6 +211,7 @@ fbOverlayCopyWindow(WindowPtr pWin, xPoint ptOldOrg, RegionPtr prgnSrc)
     /*
      * Compute the portion of each fb affected by this copy
      */
+    assert(pScrPriv->nlayers <= FB_OVERLAY_MAX);
     for (i = 0; i < pScrPriv->nlayers; i++) {
         RegionNull(&layerRgn[i]);
         RegionIntersect(&layerRgn[i], &rgnDst,