Commit eeb99329 removed an important optimization avoiding quadratic
runtime when repeatedly scanning the input buffer for terminating
characters in the HTML push parser. The related bug is
https://bugzilla.gnome.org/show_bug.cgi?id=444994
Make sure that ctxt->checkIndex is always written and store additional
parser state in ctxt->inSubset which is unused in the HTML parser.
Found by OSS-Fuzz.
