Use wxCHECKs to protect non-position TOOL_EVENTS

Getting the position of a non-position TOOL_EVENT will
now result in a wxCHECK, and a null-position (0,0) will
be returned. The new interface HasPosition() can be used
to determine if a call to a position function is safe to
make from the caller.

Fixes the underlying danger behind lp:1796045, which has
already been neutralised by a4966adb6, which avoids calling
the Position() functions when the TOOL_EVENT is not known
to be a position.

Fixes: lp:1796045
* https://bugs.launchpad.net/kicad/+bug/1796045

include/tool/tool_event.h

@@ -30,10 +30,16 @@
 #include <iterator>
 
 #include <math/vector2d.h>
-#include <cassert>
 
 #include <core/optional.h>
 
+#ifdef WX_COMPATIBILITY
+#include <wx/debug.h>
+#else
+#include <cassert>
+#endif
+
+
 class TOOL_ACTION;
 class TOOL_MANAGER;
 
@@ -236,26 +242,29 @@ public:
         return m_actions;
     }
 
+    ///> Returns if it this event has a valid position (true for mouse events)
+    bool HasPosition() const
+    {
+        return m_category == TC_MOUSE;
+    }
+
     ///> Returns information about difference between current mouse cursor position and the place
     ///> where dragging has started.
-    const VECTOR2D& Delta() const
+    const VECTOR2D Delta() const
     {
-        assert( m_category == TC_MOUSE );    // this should be used only with mouse events
-        return m_mouseDelta;
+        return returnCheckedPosition( m_mouseDelta );
     }
 
     ///> Returns mouse cursor position in world coordinates.
-    const VECTOR2D& Position() const
+    const VECTOR2D Position() const
     {
-        assert( m_category == TC_MOUSE );    // this should be used only with mouse events
-        return m_mousePos;
+        return returnCheckedPosition( m_mousePos );
     }
 
     ///> Returns the point where dragging has started.
-    const VECTOR2D& DragOrigin() const
+    const VECTOR2D DragOrigin() const
     {
-        assert( m_category == TC_MOUSE );    // this should be used only with mouse events
-        return m_mouseDragOrigin;
+        return returnCheckedPosition( m_mouseDragOrigin );
     }
 
     ///> Returns information about mouse buttons state.
@@ -432,6 +441,27 @@ private:
         m_modifiers = aMods;
     }
 
+    /**
+     * Ensure that the event is a type that has a position before returning a
+     * position, otherwise return a mull-constructed position.
+     * Used to defend the position accessors from runtime access when the event
+     * does not have a valid position.
+     *
+     * @param aPos the position to return if the event is valid
+     * @return the checked position
+     */
+    VECTOR2D returnCheckedPosition( const VECTOR2D& aPos ) const
+    {
+    #ifdef WX_COMPATIBILITY
+        wxCHECK_MSG( HasPosition(), VECTOR2D(),
+            "Attempted to get position from non-position event" );
+    #else
+        assert( HasPosition() );
+    #endif
+
+        return aPos;
+    }
+
     TOOL_EVENT_CATEGORY m_category;
     TOOL_ACTIONS m_actions;
     TOOL_ACTION_SCOPE m_scope;