net: ipv6: net_icmpv6_get_na_hdr() may return NULL

If net_icmpv6_get_na_hdr() returns NULL (which is possible with a
specially crafted packet), utility functions net_is_solicited(),
net_is_router(), and net_is_override() will attempt to read invalid
memory.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>