`next_boundary()` panics when cursor is at the end of the given chunk

[truchi]

Hello,

In prev_boundary we have this check (for the following unwrap not to panic):

if self.offset == chunk_start {
    return Err(GraphemeIncomplete::PrevChunk);
}

I suggest this in next_boundary() (for the same reason):

if self.offset == chunk_start + chunk.len() {
    return Err(GraphemeIncomplete::NextChunk);
}

Thank you!