pimd: vrf may be NULL from pim_cmd_lookup_vrf

pimd: vrf may be NULL from pim_cmd_lookup_vrf

The call into pim_cmd_lookup_vrf may be NULL
and dereferencing it before ensuring that the
vrf pointer is non-NULL is a good way to crash.

A crash can be initiated in pim:

eva# show ip msdp vrf NOEXIST mesh-group
vtysh: error reading from pimd: Permission denied (13)Warning: closing connection to pimd because of an I/O error!
eva# 2022/08/15 11:47:38 [PHJDC-499N2][EC 100663314] STARVATION: task vtysh_rl_read (560b77f76de6) ran for 16777ms (cpu time 0ms)

eva#

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