pdn: use swap in Shape::convertVectorToTree

Assignment doesn't guarantee a capacity reduction.

Signed-off-by: Matt Liberty <[email protected]>

src/pdn/src/shape.cpp

@@ -625,7 +625,8 @@ ShapeTreeMap Shape::convertVectorToTree(ShapeVectorMap& vec)
     trees[layer] = ShapeTree(vals.begin(), vals.end());
   }
 
-  vec = ShapeVectorMap();
+  ShapeVectorMap empty;
+  vec.swap(empty);
 
   return trees;
 }