Add null check in wxAuiToolBar event handler

This also prevents the AUI_TOOLBAR_BEGIN_DRAG event from
being sent if there is no active toolbar item being dragged.

Closes https://github.com/wxWidgets/wxWidgets/pull/2062

src/aui/auibar.cpp

@@ -2839,7 +2839,7 @@ void wxAuiToolBar::OnMotion(wxMouseEvent& evt)
     const bool button_pressed = HasCapture();
 
     // start a drag event
-    if (!m_dragging && button_pressed &&
+    if (!m_dragging && button_pressed && m_actionItem &&
         abs(evt.GetX() - m_actionPos.x) + abs(evt.GetY() - m_actionPos.y) > 5)
     {
         // TODO: sending this event only makes sense if there is an 'END_DRAG'