Adjust a column number in an error message

Adjust a column number in an error message
after the roff_expand() reorganization in roff.c rev. 1.388.

The new parsing direction has two effects:
1. Correct output when a line contains more than one expanding
escape sequence that has a side effect.
2. Column numbers in diagnostic messages now report the changed
column numbers after any expansions left of them have taken place;
in the past, column numbers refered to the original input line.

Arguably, item 2 was a bit better in its old state, but slightly
less helpful diagnostics are a small price to pay for correct
output.  Besides, when the expansion of user-defined strings or
macros is involved, in many cases, mandoc(1) is already unable to
report meaningful line and column numbers, so item 2 is not a
noteworthy regression.  The effort and code complication for fixing
that would probably be excessive, in particular since well-written
manual pages are not supposed to use such features in the first place.