bgpd: bgp_clear_adj_in|remove dest may be freed

bgpd: bgp_clear_adj_in|remove dest may be freed

dest will not be freed due to lock but coverity does not know
that.  Give it a hint.  This change includes modifying bgp_dest_unlock_node
to return the dest pointer so that we can determine if we should
continue working on dest or not with an assert.  Since this
is lock based we should be ok.

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