gpgparsemail: Fix case of zero length continuation lines.

* tools/rfc822parse.c (parse_field): Loop after continuation line.

Using header lines like

Name:[lf]
[space][lf]
[lf]

resulted in running into the "(s2 = strchr (delimiters2, *s)" branch
and inserting a new token for the empty continuation line. This also
led to one byte read after the string which is what Hanno figured.
The new code should handle empty continuation lines correct.

• Reported-by: Hanno Böck
• Signed-off-by: Werner Koch <wk@gnupg.org>