Check wxDir::GetNext() return value in wxFindNextFile().

This doesn't really change anything as the returned value would be empty
anyhow in case of an error, but suppresses Coverity warning about not checking
the return value of a function whose return value is checked in other places.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73143 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775