Fix wxHTMLDataObject compilation and memory leaks.

Fix wxHTMLDataObject compilation and memory leaks.

Use wxString::FromUTF8() and utf8_str() instead of directly using wxConvUTF8
as this is simpler and also works correctly in non-Unicode build (and more
efficiently in UTF-8 build as no conversion is done there).

Do not allocate -- and leak -- buffer in wxHTMLDataObject::GetDataHere(),
we're supposed to be putting data into the caller-provided buffer instead of
using our own.

Closes #14391.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71712 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775