Fix comparison between type uint16_t and wider type size_t in a loop

Fix comparison between type uint16_t and  wider type size_t in a loop

Found by LGTM.com (see below for description), and while it should not
happen as EDNS OPT RDLEN is uint16_t, the fix is easy.  A little bit
of cleanup is included too.

> In a loop condition, comparison of a value of a narrow type with a value
> of a wide type may result in unexpected behavior if the wider value is
> sufficiently large (or small). This is because the narrower value may
> overflow. This can lead to an infinite loop.