d2gamescreen/char_select: Fix crash on double clicking singleplayer

if we clicked twice on the single player button of the main menu, then
the first click would set the current screen to the
character_select. The second click directly after that will be
handled by the character_select screen. It's OnLoad() method was not
called yet, which initialized all widgets. The click handler of
character_select screen referenced the nil scrollbar and crashes.

d2game/d2gamescreen/character_select.go

@@ -47,6 +47,7 @@ type CharacterSelect struct {
 	tickTimer              float64
 	storedTickTimer        float64
 	showDeleteConfirmation bool
+	loaded                 bool
 	connectionType         d2clientconnectiontype.ClientConnectionType
 	connectionHost         string
 
@@ -195,6 +196,7 @@ func (v *CharacterSelect) OnLoad(loading d2screen.LoadingState) {
 	}
 
 	v.refreshGameStates()
+	v.loaded = true
 }
 
 func (v *CharacterSelect) loadBackground() {
@@ -429,6 +431,10 @@ func (v *CharacterSelect) moveSelectionBox() {
 
 // OnMouseButtonDown is called when a mouse button is clicked
 func (v *CharacterSelect) OnMouseButtonDown(event d2interface.MouseEvent) bool {
+	if !v.loaded {
+		return false
+	}
+
 	if v.showDeleteConfirmation {
 		return false
 	}
@@ -555,5 +561,7 @@ func (v *CharacterSelect) OnUnload() error {
 		return err
 	}
 
+	v.loaded = false
+
 	return nil
 }