pim6d: Fix crash in ipv6 pim command

pim6d: Fix crash in ipv6 pim command

Problem:
Execute the below commands, pim6d core happens.
interface ens193
 ip address 69.0.0.2/24
 ipv6 address 8000::1/120
 ipv6 mld
 ipv6 pim
We see crash only if the interface is not configured, and
we are executing PIM/MLD commands.

RootCause:
Interface ens193 is not configured. So, it will have
ifindex = 0 and mroute_vif_index = -1.
Currently, we don't enable MLD on an interface if
mroute_vif_index < 0. So, pim_ifp->MLD = NULL.
In the API pim_if_membership_refresh(), we are accessing
pim_ifp->MLD NULL pointer which leads to crash.

Fix:
Added NULL check before accessing pim_ifp->MLD pointer in
the API pim_if_membership_refresh().

Issue: #13385

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