Actually handle the yytable zero value correctly this time.

Actually handle the yytable zero value correctly this time.

* data/bison.m4 (b4_integral_parser_tables_map): Don't mention
zero values in the YYTABLE comments.
* data/glr.c (yytable_value_is_error): Don't check for zero
value.
* data/lalr1.cc (yy_table_value_is_error_): Likewise.
* data/yacc.c (yytable_value_is_error): Likewise.
* data/lalr1.java (yy_table_value_is_error_): Likewise.
(yysyntax_error): Fix typo in code: use yytable_ not yycheck_.
* src/tables.h: In header comments, explain why it's useless to
check for a zero value in yytable.