Core/Movement: move MoveSplineInit (#21857)

The number of edge cases in which weirdness is seen on "effect movements" will be kinda reduced, plus consistency, plus movementInform on custom movement spline initalizations.

Core/Movement: use helpers for validation

Core/Movement: crashfix workaround

src/server/game/AI/SmartScripts/SmartScriptMgr.cpp

@@ -862,7 +862,7 @@ bool SmartAIMgr::IsEventValid(SmartScriptHolder& e)
             }
             case SMART_EVENT_MOVEMENTINFORM:
             {
-                if (e.event.movementInform.type >= MAX_MOTION_TYPE)
+                if (IsInvalidMovementGeneratorType(e.event.movementInform.type))
                 {
                     TC_LOG_ERROR("sql.sql", "SmartAIMgr: Entry %d SourceType %u Event %u Action %u uses invalid Motion type %u, skipped.", e.entryOrGuid, e.GetScriptType(), e.event_id, e.GetActionType(), e.event.movementInform.type);
                     return false;