func-arity minor improvements

func-arity minor improvements

* src/bytecode.c (get_byte_code_arity): Omit unnecessary
runtime test for integer argument, unless debugging.
Use EMACS_INT for Emacs integers.
* src/eval.c (Ffunc_arity): Omit unused locals.
Avoid side effects in ‘if’ expr.
(lambda_arity): Use bool for boolean, and EMACS_INT for Emacs ints.