Hint out check for unsigned negative values.

On FreeBSD socklen_t is unsigned so the check negative len
in inet6_opt_append() is redundant and likely to be optimized
away by the compiler.

On other operating systems this is not necessarily so, and
in the future we may want to sign it so leave the check in
but place it in a secondary position as a subtle indication
that the bogus check is intentional.

Discussed with: rpaulo

CID: 1017783