GraphemeCursor::prev_boundary() crashes when the current position is at the beginning of the chunk

[jmuk]

It'd be good if the following code works.

let s = "abcd";
let mut cursor = GraphemeCursor::new(2, s.len(), true);
assert_eq!(cursor.prev_boundary(&s[2..], 2), Err(GraphemeIncomplete::PrevChunk));
assert_eq!(cursor.prev_boundary(&s[..2], 2), Ok(Some(1));

Currently it crashes during the first assertion, since prev_boundary expects at least there's one character from the chunk start to the cursor position.

[jmuk]

Seems missed to be losed, but #40 fixed this.