Only set a simplewindow's opacity if there is a window

Technically setting the opacity of an invalid window doesn't hurt, but
still it's nicer to catch this instead.

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

swindow.h

@@ -88,6 +88,7 @@ static inline void
 simplewindow_opacity_set(simple_window_t *sw, double opacity)
 {
     sw->opacity = opacity;
+    if(sw->window != XCB_NONE)
         window_opacity_set(sw->window, opacity);
 }