Fix two places in the ICMP6 code where we could dereference a NULL pointer
in the icmp6_input() function.

When processing an ICMP6_ECHO_REQUEST, if IP6_EXTHDR_GET fails, it will
set nicmp6 and n to NULL. Therefore, we should condition our modification
to nicmp6 on n being not NULL.

And, when processing an ICMP6_WRUREQUEST in the (mode != FQDN) case, if
m_dup_pkthdr() fails, the code will set n to NULL. However, the very next
line dereferences n. Therefore, when m_dup_pkthdr() fails, we should
discontinue further processing and follow the same path as when m_gethdr()
fails.

Reported by: clang static analyzer
Reviewed by: ae
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D10941