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.

src/scan-code.l

@@ -81,7 +81,7 @@ static bool untyped_var_seen;
    historically almost any character is allowed in a tag.  We disallow
    NUL and newline, as this simplifies our implementation.  We allow
    "->" as a means to dereference a pointer.  */
-tag      ([^\0\n>]|->)+
+tag      ([^\0\n>]|->)*[^-]
 
 /* Zero or more instances of backslash-newline.  Following GCC, allow
    white space between the backslash and the newline.  */