pimd: Fix Null pointer dereferences

pimd: Fix Null pointer dereferences

Fixing the below problem:
Dereferencing a pointer that might be "NULL" "group_dnode"
when calling "yang_is_last_list_dnode" in api pim_process_no_rp_cmd

Although there is no NULL pointer dereference since
yang_dnode_exists is called before using the dnode.
So removing the unnecessary yang_dnode_exists api call
and directly get the node and if node does not exists,
return.

Signed-off-by: Mobashshera Rasool <mrasool@vmware.com>