ping6: quiet an undefined variable warning

GCC 4.2 isn't smart enough to know that this variable is already defined by
the time it's used.

Submitted by: Ján Sučan <sucanjan@gmail.com>
MFC after: 2 weeks
Sponsored by: Google, inc. (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21269