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.

THANKS

@@ -100,6 +100,7 @@ Lie Yan                   lie.yan@kaust.edu.sa
 Magnus Fromreide          magfr@lysator.liu.se
 Marc Autret               autret_m@epita.fr
 Marc Mendiola             mmendiol@usc.edu
+Marc Schönefeld           marc.schoenefeld@gmx.org
 Mark Boyall               wolfeinstein@gmail.com
 Martin Jacobs             martin.jacobs@arcor.de
 Martin Mokrejs            mmokrejs@natur.cuni.cz

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

tests/input.at

@@ -624,7 +624,7 @@ AT_SETUP([Symbol redeclared])
 AT_DATA([[input.y]],
 [[%token FOO FOO
 %token BAR 12 BAR 12
-
+%token EOF 0 EOF 0
 %%
 exp: FOO BAR
 ]])
@@ -642,6 +642,12 @@ input.y:2.15-17: warning: symbol BAR redeclared [-Wother]
 input.y:2.8-10: previous declaration
     2 | %token BAR 12 BAR 12
       |        ^~~
+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
+      |        ^~~
 ]])
 
 AT_CLEANUP