Fix precedence for end token.

Since Bison 2.3b, which restored the ability of precedence
directives to assign user token numbers, doing so for user token
number 0 has produced an assertion failure.
* NEWS (2.5): Document fix.
* src/symtab.c (symbol_user_token_number_set): In the case of the
end token, don't decrement ntokens if it was never incremented.
* tests/regression.at (Token number in precedence declaration):
Extend.
(cherry picked from commit 9d6af15318)

src/symtab.c

@@ -403,10 +403,11 @@ symbol_user_token_number_set (symbol *sym, int user_token_number, location loc)
   if (user_token_number == 0)
     {
       endtoken = sym;
-      endtoken->number = 0;
       /* It is always mapped to 0, so it was already counted in
 	 NTOKENS.  */
-      --ntokens;
+      if (endtoken->number != NUMBER_UNDEFINED)
+        --ntokens;
+      endtoken->number = 0;
     }
 }