Correctly handle S_FALSE return value of IActiveMovie::get_Duration().

Correctly handle S_FALSE return value of IActiveMovie::get_Duration().

IActiveMovie::get_Duration() can return S_FALSE in which case outDuration
isn't initialized and so wxAMMediaBackend::GetDuration() would return a
completely wrong value.

Fix this by returning 0 from it instead which seems like the only reasonable
thing to do (in the absence of documentation of this interface it's not really
clear what does S_FALSE return value mean nor why didn't it return it before).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65845 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775