Fix #repeat(0,0) bug

`str("a").repeat(0, 0)` would successfully parse `"a"`, though only 0 repetitions were asked for. Now it rejects that input.

I noticed this bug due to this failing test in a Parslet example I’m writing: https://github.com/roryokane/parslet/blob/new-examples/example/count_captured_chars.rb#L39.