Fix bug with non-NUL-terminaed inputs in wxMBConvUTF8.

We read beyond the provided maximal length as we didn't update the remaining
length while parsing the remaining bytes of an UTF-8-encoded code point.

Fix this and add a test for it.

Closes #15901.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75733 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775