Fix byte-compilation error

The specific error:

```
smartparens.el:5009:1:Error: Wrong number of arguments: (lambda (vars-vals then &optional else) "If all VALS evaluate to true, bind them to their corresponding
VARS and do THEN, otherwise do ELSE. VARS-VALS should be a list
of (VAR VAL) pairs (corresponding to the bindings of `let*')." (let ((first-pair (car vars-vals)) (rest (cdr vars-vals))) (if (= (length vars-vals) 1) (list (quote -if-let) first-pair then else) (list (quote -if-let) first-pair (list (quote -if-let*) rest then else) else)))), 4
```

Also, there might have been a minor bug in that before this patch,
I think Smartparens would _always_ execute `(message "This operation would…` after running `sp-slurp-hybrid-sexp`.