Fixes for parsing invalid HTML without tag ends.

Fixes for parsing invalid HTML without tag ends.

The code in wxHtmlParser supposed in many places that a '<' character must be
always followed by a '>' one and could create (and sometimes dereference)
invalid iterators if this wasn't the case resulting in asserts from MSVC debug
CRT and possibly crashes.

Fix this by ensuring that only valid iterators are used and add a trivial unit
test for wxHtmlParser which checks that it can parse invalid HTML without
crashing.

Closes #12869.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66678 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775