avoid panic: entry[pos..] => entry.get(pos..)

[lengyijun]

No description provided.

[gwenn]

I may be wrong but:
We have:

        if line.is_empty() || pos < line.len() {
            return None;
        }

So pos == line.len() && pos > 0.
So I guess that the panic can happen only if history returns an entry that doesn't start with line.
Which seems fine to me.