Fix lexing failure

[314eter]

When there is an invalid token consisting of one character followed by eof, long_error tries to print more than one character, which results in a failure.

# Yojson.Safe.from_string "x";;
Exception: Failure "lexing: empty token".

# Yojson.Safe.from_string "xx";;
Exception: Yojson.Json_error "Line 1, bytes 0-2:\nInvalid token 'xx'".

I don't know whether there was a valid reason to look for more characters. If not, the fix is very easy.

[mjambon]

We're only using the junk pattern and rule for error reporting so this should work.

[mjambon]

opam release in progress ocaml/opam-repository#7691