Fix comment and tweak eval_sub

Fix comment and tweak eval_sub

* src/eval.c (eval_sub): Check whether Fassq returns Qnil,
not whether it returns a cons, as NILP is faster than CONSP
nowadays.  Remove incorrect comment “only original_fun and
original_args have values that will be used below”; instead,
move declarations around so that the set of variables with
useful values is obvious.