Previously panic occured on a `let*` with an empty body, due to how
`let_star` and `eval` functions interacted:
- (let* (a 1)) resulted in an index-out-of-bound panic on trying to
evaluate the empty body of the statement, a simple guard has been
inserted, I probably can write it a bit better
Signed-off-by: teo3300 <matteo.rogora@live.it>