code: be robust to reference with invalid tags

Because we want to support $<a->b>$, we must accept -> in type tags,
and reject $<->$, as it is unfinished.
Reported by Ahcheong Lee.

* src/scan-code.l (yylex): Make sure "tag" does not end with -, since
-> does not close the tag.
* tests/input.at (Stray $ or @): Check this.

THANKS

@@ -4,8 +4,9 @@ it is today without the invaluable help of these people:
 Aaro Koskinen             aaro.koskinen@iki.fi
 Аскар Сафин               safinaskar@mail.ru
 Adam Sampson              ats@offog.org
+Ahcheong Lee              dkcjd2000@gmail.com
 Airy Andre                Airy.Andre@edf.fr
-Akim Demaille             akim@lrde.epita.fr
+Akim Demaille             akim@gnu.org
 Albert Chin-A-Young       china@thewrittenword.com
 Alexander Belopolsky      alexb@rentec.com
 Alexandre Duret-Lutz      adl@lrde.epita.fr