reader: reject rules on YYEOF

We crashed when rules were given on YYEOF.
Reported by Han Zheng.
Fixes https://github.com/akimd/bison/issues/92.

* src/reader.c (check_and_convert_grammar): Make sure YYEOF is not
an nterm.
* tests/input.at (Rule for YYEOF): New.
2 files changed