Fix freed-memory crasher.

While the DRC_ENGINE is not long-lived, it's test providers are,
and must be cleared for each use.

pcbnew/drc/drc_rtree.h

@@ -1,7 +1,7 @@
 /*
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
- * Copyright (C) 2019 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 2020 KiCad Developers, see AUTHORS.txt for contributors.
  * Copyright (C) 2020 CERN
  *
  * This program is free software; you can redistribute it and/or
@@ -82,7 +82,7 @@ public:
 
     /**
      * Function Insert()
-     * Inserts an item into the tree. Item's bounding box is taken via its GetBoundingBox() method.
+     * Inserts an item into the tree.
      */
     void insert( BOARD_ITEM* aItem, int aWorstClearance = 0, int aLayer = UNDEFINED_LAYER )
     {

pcbnew/drc/drc_test_provider_copper_clearance.cpp

@@ -27,7 +27,6 @@
 #include <class_pad.h>
 #include <class_track.h>
 
-#include <geometry/shape_arc.h>
 #include <geometry/seg.h>
 #include <geometry/shape_poly_set.h>
 #include <geometry/shape_rect.h>
@@ -144,6 +143,8 @@ bool DRC_TEST_PROVIDER_COPPER_CLEARANCE::Run()
     size_t count = 0;
     size_t ii = 0;
 
+    m_copperTree.clear();
+
     auto countItems =
             [&]( BOARD_ITEM* item ) -> bool
             {

pcbnew/drc/drc_test_provider_hole_clearance.cpp

@@ -117,6 +117,8 @@ bool DRC_TEST_PROVIDER_HOLE_CLEARANCE::Run()
     size_t       count = 0;
     size_t       ii = 0;
 
+    m_holeTree.clear();
+
     auto countItems =
             [&]( BOARD_ITEM* item ) -> bool
             {