Core/Maps: Replace some Instanceable() checks with IsDungeon()

Port From (https://github.com/TrinityCore/TrinityCore/commit/8466894d816f484f30cdbe47b980a142870c955e)

Source/Game/Maps/Instances/InstanceSaveManager.cs

@@ -454,7 +454,7 @@ namespace Game.Maps
         {
             Log.outDebug(LogFilter.Maps, "InstanceSaveMgr._ResetInstance {0}, {1}", mapid, instanceId);
             Map map = Global.MapMgr.CreateBaseMap(mapid);
-            if (!map.Instanceable())
+            if (!map.IsDungeon())
                 return;
 
             var pair = m_instanceSaveById.Find(instanceId);
@@ -464,12 +464,9 @@ namespace Game.Maps
             DeleteInstanceFromDB(instanceId);                       // even if save not loaded
 
             Map iMap = ((MapInstanced)map).FindInstanceMap(instanceId);
-
-            if (iMap != null && iMap.IsDungeon())
-                ((InstanceMap)iMap).Reset(InstanceResetMethod.RespawnDelay);
-
             if (iMap != null)
             {
+                ((InstanceMap)iMap).Reset(InstanceResetMethod.RespawnDelay);
                 iMap.DeleteRespawnTimes();
                 iMap.DeleteCorpseData();
             }

Source/Game/Maps/Instances/MapInstance.cs

@@ -82,7 +82,7 @@ namespace Game.Maps
             if (GetId() != mapId || player == null)
                 return null;
 
-            Map map;
+            Map map = null;
             uint newInstanceId;                       // instanceId of the resulting map
 
             if (IsBattlegroundOrArena())
@@ -106,7 +106,7 @@ namespace Game.Maps
                     }
                 }
             }
-            else if(!IsGarrison())
+            else if(IsDungeon())
             {
                 InstanceBind pBind = player.GetBoundInstance(GetId(), player.GetDifficultyID(GetEntry()));
                 InstanceSave pSave = pBind != null ? pBind.save : null;
@@ -162,7 +162,7 @@ namespace Game.Maps
                         map = CreateInstance(newInstanceId, null, diff, player.GetTeamId());
                 }
             }
-            else
+            else if (IsGarrison())
             {
                 newInstanceId = (uint)player.GetGUID().GetCounter();
                 map = FindInstanceMap(newInstanceId);