wg: use key_is_zero for comparing to zeros

wg: use key_is_zero for comparing to zeros

Maybe an attacker on the system could use the infoleak in /proc to gauge
how long a wg(8) process takes to complete and determine the number of
leading zeros. This is somewhat ridiculous, but it's possible somebody
somewhere might at somepoint care in the future, so alright.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>