net: iface: Add NULL pointer check in net_if_ipv6_set_reachable_time

In case the IPv6 context pointer was not set on an interface (for
instance due to IPv6 context shortage), processing the RA message could
lead to a crash (i. e. NULL pointer dereference). Protect against this
by adding NULL pointer check, similarly to other functions in this area.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>