Protect polygon build

If we don't have corners, then we cannot add any points or lines

(cherry picked from commit f33fd675d2)

common/tool/point_editor_behavior.cpp

@@ -31,6 +31,9 @@ void POLYGON_POINT_EDIT_BEHAVIOR::BuildForPolyOutline( EDIT_POINTS&          aPo
 {
     const int cornersCount = aOutline.TotalVertices();
 
+    if( cornersCount == 0 )
+        return;
+
     for( auto iterator = aOutline.CIterateWithHoles(); iterator; iterator++ )
     {
         aPoints.AddPoint( *iterator );