Fix crash in wxMediaCtrl::GetDownloadProgress() with ActiveMovie backend.

Fix crash in wxMediaCtrl::GetDownloadProgress() with ActiveMovie backend.

IActiveMovie::get_FilterGraph() may return S_FALSE, i.e. succeed, while
leaving the output pointer NULL. Check for this and don't dereference the
pointer before checking for its validity.

Also do it for the other call in this function just in case. And rearrange the
code to be slightly more readable.

Closes #11894.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63831 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775