Pcbnew: fix Bug #1404191 (Zone filling fails when a zone inside having the same net and a highter priority exists, which is allowed to manage different fill parameters )

polygon/PolyLine.cpp

@@ -1501,7 +1501,7 @@ void ConvertPolysListWithHolesToOnePolygon( const CPOLYGONS_LIST& aPolysListWith
     // If polycount<= 1, there is no holes found, and therefore just copy the polygon.
     if( polycount <= 1 )
     {
-        aOnePolyList = aPolysListWithHoles;
+        aOnePolyList.Append( aPolysListWithHoles );
         return;
     }