Don't read beyond the end of input buffer when decoding UTF-16.

Don't read beyond the end of input buffer when decoding UTF-16.

wxMBConvStrictUTF8::FromWChar() didn't update the input length correctly when
encountering a surrogate while decoding UTF-16 and could read beyond the end
of the input buffer in this case.

Fix this by simply adjusting the input length when a surrogate is read.

Closes #13614.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69676 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775