zebra: can't improve efficiency for recursive depends

zebra: can't improve efficiency for recursive depends

cb86eba3ab3d82f540bdb9ed5f65d361ca301ea8 was causing zebra to crash
when handling a nexthop group that had a nexthop which was recursively resolved.

Steps to recreate:

!
nexthop-group red
 nexthop 1.1.1.1
 nexthop 1.1.1.2
!

sharp install routes 8.8.8.1 nexthop-group red 1

=========================================
==11898== Invalid write of size 8
==11898==    at 0x48E53B4: _nexthop_add_sorted (nexthop_group.c:254)
==11898==    by 0x48E5336: nexthop_group_add_sorted (nexthop_group.c:296)
==11898==    by 0x453593: handle_recursive_depend (zebra_nhg.c:481)
==11898==    by 0x451CA8: zebra_nhg_find (zebra_nhg.c:572)
==11898==    by 0x4530FB: zebra_nhg_find_nexthop (zebra_nhg.c:597)
==11898==    by 0x4536B4: depends_find (zebra_nhg.c:1065)
==11898==    by 0x453526: depends_find_add (zebra_nhg.c:1087)
==11898==    by 0x451C4D: zebra_nhg_find (zebra_nhg.c:567)
==11898==    by 0x4519DE: zebra_nhg_rib_find (zebra_nhg.c:1126)
==11898==    by 0x452268: nexthop_active_update (zebra_nhg.c:1729)
==11898==    by 0x461517: rib_process (zebra_rib.c:1049)
==11898==    by 0x4610C8: process_subq_route (zebra_rib.c:1967)
==11898==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Zebra crashes because we weren't handling the case of the depend nexthop
being recursive.

For this case, we cannot make the function more efficient. A nexthop
could resolve to a group of any size, thus we need allocs/frees.

To solve this and retain the goal of the original patch, we separate out the
two cases so it will still be more efficient if the nexthop is not recursive.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>