`Witnes::taproot_control_block` returns `Some(signature)` for key spends

We need to have a check that the `Witness` is actually a script spend (at least 2 witness elements).

~~There's same bug in `tapscript` method but that one is also broken for other reasons.~~ actually, it was using `third_to_last` which returned `None` if there were 2 witness elements. But the structure of the code looked the same and was confusing.