fixed yet another bug in wxStream classes

(This time, the braindead wannabe coder who wrote the original code created
wxStreamBase instance and then casted it to wxInputStream and hapilly used the stream,
which could of course never work. Anybody knows a cheap hitman?)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@11440 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775