Use wxASSERT() instead of assert() in wx code.

Use wxASSERT() instead of assert() in wx code.

This is more consistent and ensures that all asserts in wxWidgets are
controlled by a single wxDEBUG_LEVEL setting instead of also relying on NDEBUG
as standard assert() macro does.

See #11155.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61959 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775