[Groff] grohtml fix

[Gaius Mulley]

Hi,

a small note to say that I've checked in a fix to grohtml:

        * src/devices/grohtml/post-html.cpp (html_printer::flush_page):
        added call to flush_text to flush any outstanding html tags
        on the paragraph stack.

which fixes a problem that if a `.bp' occurred during a <pre> block
(.nf \fC) then the final </pre> (any other outstanding tags) would be
omitted.

Gaius