Prevent crash if the controller didn't handle TOUCH_BEGIN

Fixes http://bugs.sugarlabs.org/ticket/4024

Signed-off-by: Carlos Garnacho <carlos@lanedo.com>
Acked-by: Simon Schampijer <simon@laptop.org>

src/sugar3/event-controller/sugar-touch-controller.c

@@ -129,8 +129,14 @@ sugar_touch_controller_handle_event (SugarEventController *controller,
       break;
     case GDK_TOUCH_UPDATE:
       point = g_hash_table_lookup (priv->touches, sequence);
-      point->x = event->touch.x;
-      point->y = event->touch.y;
+
+      if (point)
+        {
+          point->x = event->touch.x;
+          point->y = event->touch.y;
+        }
+      else
+        handled = FALSE;
       break;
     default:
       handled = FALSE;