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.
(cherry picked from commit a9bd6f6e)