pimd: Fix pim_mroute_del crash while terminating pimd

pimd: Fix pim_mroute_del crash while terminating pimd

When pimd is getting terminated, pim_upstream_del() gets called as
part of cleaning process. pim_upstream_del() deletes the route and
assigns NULL to the up->channel_oil. It also deletes each if_channel
by calling the function pim_ifchannel_delete().
pim_ifchannel_delete() internally calls the caller function pim_upstream_del(),
if it is the last ifchannel for that upstream. So pim_upstream_del
is getting called twice, which will access the up->channel_oil which
was already set to NULL before. This results in crash.

Fix:
pim_ifchannel_delete() should call pim_upstream_del (caller function)
only if the up->ref_count > 0. Added an assert(up->ref_count > 0) in
the function pim_upstream_del().

Signed-off-by: Sarita Patra <saritap@vmware.com>