fix: don't die when EOF token is defined twice

With

    %token EOF 0 EOF 0

we get

    input.y:3.14-16: warning: symbol EOF redeclared [-Wother]
        3 | %token EOF 0 EOF 0
          |              ^~~
    input.y:3.8-10: previous declaration
        3 | %token EOF 0 EOF 0
          |        ^~~
    Assertion failed: (nsyms == ntokens + nvars), function check_and_convert_grammar,
        file /Users/akim/src/gnu/bison/src/reader.c, line 839.

Reported by Marc Schönefeld.

* src/symtab.c (symbol_user_token_number_set): Register only the
first definition of the end of input token.
* tests/input.at (Symbol redeclared): Check that case.

src/symtab.c

@@ -495,7 +495,7 @@ symbol_user_token_number_set (symbol *sym, int user_token_number, location loc)
     {
       *user_token_numberp = user_token_number;
       /* User defined $end token? */
-      if (user_token_number == 0)
+      if (user_token_number == 0 && !endtoken)
         {
           endtoken = sym->content->symbol;
           /* It is always mapped to 0, so it was already counted in