XCB: Move the assert from 5a0f8f7

Calling _cairo_xcb_surface_ensure_picture() on a XCB surface whose fallback
member is non-null is always an error. It's possible that the surface first gets
a picture assigned and later it's fallback member is set. In this situation,
it's still wrong to use the surface's picture for any drawing-

Signed-off-by: Uli Schlachter <psychon@znc.in>

src/cairo-xcb-surface-render.c

@@ -300,8 +300,8 @@ _cairo_xcb_surface_clear_clip_region (cairo_xcb_surface_t *surface)
 static void
 _cairo_xcb_surface_ensure_picture (cairo_xcb_surface_t *surface)
 {
+    assert (surface->fallback == NULL);
     if (surface->picture == XCB_NONE) {
-	assert (surface->fallback == NULL);
 	surface->picture = _cairo_xcb_connection_get_xid (surface->connection);
 	_cairo_xcb_connection_render_create_picture (surface->connection,
 						     surface->picture,