Fix edge case crash in terrain line painting functionality

Author: Rampastring

src/TSMapEditor/Constants.cs

@@ -4,7 +4,7 @@ namespace TSMapEditor
 {
     public static class Constants
     {
-        public const string ReleaseVersion = "1.8.6";
+        public const string ReleaseVersion = "1.8.7";
 
         public static int CellSizeX = 48;
         public static int CellSizeY = 24;

src/TSMapEditor/UI/CursorActions/PlaceTerrainCursorAction.cs

@@ -280,6 +280,10 @@ protected override Mutation CreateLinePlacementMutation(Direction direction, int
         protected override void ApplyLine(Point2D cellCoords)
         {
             var adjustedCellCoords = GetAdjustedCellCoords(cellCoords);
+
+            if (adjustedCellCoords == LineSourceCell.Value)
+                return;
+
             (Direction direction, int length) = GetLineInformation(adjustedCellCoords);
             var mutation = CreateLinePlacementMutation(direction, length);
             PerformMutation(mutation);