named references: fix double free.

In `rhs[name]: "a" | "b"', do not free "name" twice.
Reported by Tys Lefering.
<http://lists.gnu.org/archive/html/bug-bison/2010-06/msg00002.html>

	* src/named-ref.h, src/named-ref.c (named_ref_copy): New.
	* src/parse-gram.y (current_lhs): Rename as...
	(current_lhs_symbol): this.
	(current_lhs): New function.  Use it to free the current lhs
	named reference.
	* src/reader.c: Bind lhs to a copy of the current named reference.
	* src/symlist.c: Rely on free (0) being valid.
	* tests/named-refs.at: Test this.

(cherry picked from commit 8f462efe92)

Conflicts:

	src/parse-gram.y

src/named-ref.c

@@ -33,6 +33,12 @@ named_ref_new (uniqstr id, location loc)
   return res;
 }
 
+named_ref *
+named_ref_copy (const named_ref *r)
+{
+  return named_ref_new (r->id, r->loc);
+}
+
 void
 named_ref_free (named_ref *r)
 {