zebra: prevent crash when nhlfe is NULL

zebra: prevent crash when nhlfe is NULL

There exists a call path where the nhlfe_alloc can return NULL
for blackhole nexthops.  In this case we were still trying
to save the nhlfe pointer causing a crash when we attempted
to add it to a self-contained list.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>