ip6_output: Initialize plen for ip6_hopopts_input.

This funny little block in ip6_process_hopopts assumes it is
initialized as and behaves differently depending on whether it's zero
or not:

https://nxr.netbsd.org/xref/src/sys/netinet6/ip6_input.c?r=1.227#976

In the other call site, it is initialized to ip6->ip6_plen:

https://nxr.netbsd.org/xref/src/sys/netinet6/ip6_input.c?r=1.227#561

Reported-by: syzbot+587e3b707bdfe533283f@syzkaller.appspotmail.com
https://syzkaller.appspot.com/bug?extid=587e3b707bdfe533283f

(riastradh)