egl: added cpp assertions/sanity checks

src/gallium/winsys/egl_xlib/sw_winsys.c

@@ -194,6 +194,8 @@ surface_alloc_storage(struct pipe_winsys *winsys,
    surf->pitch = round_up(width, alignment / surf->cpp);
    surf->usage = flags;
 
+   assert(surf->cpp >= 1);
+   assert(surf->cpp <= 16);
    assert(!surf->buffer);
    surf->buffer = winsys->buffer_create(winsys, alignment,
                                         PIPE_BUFFER_USAGE_PIXEL,