[9445] Possible fix often crashes in waypoint movegen.

Thanks to DrKLO for research source of problem!

src/game/MovementGenerator.cpp

@@ -21,3 +21,10 @@
 MovementGenerator::~MovementGenerator()
 {
 }
+
+bool MovementGenerator::IsActive( Unit& u )
+{
+    // When movement generator list modified from Update movegen object erase delayed,
+    // so pointer still valid and be used for check
+    return !u.GetMotionMaster()->empty() && u.GetMotionMaster()->top() == this;
+}