hw/xwin: Ignore WM_DISPLAYCHANGE messages with 0 bpp

Ignore WM_DISPLAYCHANGE messages which indicate bpp is changing to 0.
That has no defined meaning I can find, but some graphics card drivers
appear to generate it on suspend/resume or screensaver activate/deactivate.

Signed-off-by: Colin Harrison <colin.harrison@virgin.net>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>

hw/xwin/winwndproc.c

@@ -182,6 +182,10 @@ winWindowProc (HWND hwnd, UINT message,
 	      "new height: %d new bpp: %d\n",
 	      LOWORD (lParam), HIWORD (lParam), wParam);
 
+      /* 0 bpp has no defined meaning, ignore this message */
+      if (wParam == 0)
+        break;
+
       /*
        * Check for a disruptive change in depth.
        * We can only display a message for a disruptive depth change,