html-ostream: Fix memory leak.
* gnulib-local/lib/html-ostream.oo.c: Include minmax.h.
(verify_invariants, shrink_class_stack): New functions.
(emit_pending_spans): Use them.
(html_ostream::write_mem): Shrink class stack during newline processing.
(html_ostream::free, html_ostream::begin_span): Verify invariants.
(html_ostream::end_span): Likewise. Shrink class stack.
* gnulib-local/modules/html-ostream (Depends-on): Add 'minmax'.
