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/branches/WX_3_0_BRANCH@75728 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775