[CFT][patch] mandoc: don't segfault on empty tbl(1) continuation blocks

Good day.

Attached is the patch that makes built-in tbl(1) processor in mandoc
to avoid dumping core when it renders the table with empty "T{ T}"
block and horizontally-ruled table.

The simplest way to reproduce the issue is to either
 - run 'man notmuch-config' with mail/notmuch installed;
 - run 'mandoc tests/empty-table-cdata.1' against the attached
   test-only manpage.

With the patch applied, one can utilize 'make check': regression
test was added.  Perhaps an invocation of
{{{
mtree -deU -f /usr/src/etc/mtree/BSD.tests.dist -p /usr/tests
}}}
will be needed to run 'make check' without remaking/installing
the world.

The patch is for the fresh -CURRENT.  Be interested in any results
of its application and usage.

Thanks!

P.S.: please, CC me: I am not subscribed to the list.
-- 
Eygene Ryabinkin                                        ,,,^..^,,,
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