DeskBar: use the prepared transparent icon when necessary

In the odd case where there is no icon for an app, and the generic
3 boxes icon is also unavailable/removed, Deskbar now properly draws
the transparent icon that was prepared, but not used (and leaked).

src/apps/deskbar/BarApp.cpp

@@ -969,7 +969,7 @@ TBarApp::FetchAppIcon(BarTeamInfo* barInfo)
 	}
 
 	delete barInfo->iconCache[index];
-	barInfo->iconCache[index] = NULL;
+	barInfo->iconCache[index] = barInfo->icon = icon;
 }