memleak fix

engine/gamestate/gamestate.go

@@ -21,6 +21,7 @@ func (g *GameState) Do(f func()) {
 	done := make(chan struct{}, 1)
 	g.Mainfunc <- func() {
 		f()
+		f = nil //zero pointer for closure function
 		done <- struct{}{}
 	}
 	<-done