Fix out of bounds access when tokenising String_with_vars

[rgrinberg]

When the current character is '$' we peek at the next character. But we first
need to make sure that there is a next character.

[dra27]

All obviously good. Two thoughts while we're here:

• I don't think there's a way to escape a $ (unless ${$} or $($) is one... I forget).
• Given that, should $ at the end of a string be an error (but rather better than an indexing error)?

[rgrinberg]

David Allsopp <notifications@github.com> writes:

All obviously good. Two thoughts while we're here: - I don't think there's a way to escape a `$` (unless `${$}` or `$($)` is one... I forget).

Good point. I suppose we can some sort of escaping mechanism, but it seems to be very low priority as I doubt someone will run into this.

- Given that, should `$` at the end of a string be an error (but rather better than an indexing error)?

OK, I will change it to an indexing error.

[rgrinberg]

Given that, should $ at the end of a string be an error (but rather better than an indexing error)?

Wait, I'm having second thoughts about this. $ is treated literally unless { or ( follows. Why should end of string be treated any differently?

[dra27]

That's a very good point!!