Return NULL from wxWindow::GetCapture() when the capture is being lost.

Return NULL from wxWindow::GetCapture() when the capture is being lost.

Make GetCapture() return NULL during wxEVT_MOUSE_CAPTURE_{LOST,CHANGED} events
processing in wxMSW to help their handlers to avoid calling ReleaseCapture():
this shouldn't be done as the mouse capture is being lost anyhow and results
in reentrancy problems between NotifyCaptureLost() and ReleaseCapture() if
it's called.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74678 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775