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