zebra: Fix nexthop group memory leak

Address Sanitizer found this:

=================================================================
==418623==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 128 byte(s) in 4 object(s) allocated from:
    #0 0x4bd732 in calloc (/usr/lib/frr/zebra+0x4bd732)
    #1 0x7feaeab8f798 in qcalloc /home/sharpd/frr8/lib/memory.c:116:27
    #2 0x7feaeaba40f4 in nexthop_group_new /home/sharpd/frr8/lib/nexthop_group.c:270:9
    FRRouting#3 0x56859b in netlink_route_change_read_unicast /home/sharpd/frr8/zebra/rt_netlink.c:950:9
    FRRouting#4 0x5651c2 in netlink_route_change /home/sharpd/frr8/zebra/rt_netlink.c:1204:2
    FRRouting#5 0x54af15 in netlink_information_fetch /home/sharpd/frr8/zebra/kernel_netlink.c:407:10
    FRRouting#6 0x53e7a3 in netlink_parse_info /home/sharpd/frr8/zebra/kernel_netlink.c:1184:12
    FRRouting#7 0x548d46 in kernel_read /home/sharpd/frr8/zebra/kernel_netlink.c:501:2
    FRRouting#8 0x7feaeacc87f6 in thread_call /home/sharpd/frr8/lib/thread.c:2006:2
    FRRouting#9 0x7feaeab36503 in frr_run /home/sharpd/frr8/lib/libfrr.c:1198:3
    FRRouting#10 0x550d38 in main /home/sharpd/frr8/zebra/main.c:476:2
    FRRouting#11 0x7feaea492d09 in __libc_start_main csu/../csu/libc-start.c:308:16

Indirect leak of 576 byte(s) in 4 object(s) allocated from:
    #0 0x4bd732 in calloc (/usr/lib/frr/zebra+0x4bd732)
    #1 0x7feaeab8f798 in qcalloc /home/sharpd/frr8/lib/memory.c:116:27
    #2 0x7feaeab9b3f8 in nexthop_new /home/sharpd/frr8/lib/nexthop.c:373:7
    FRRouting#3 0x56875e in netlink_route_change_read_unicast /home/sharpd/frr8/zebra/rt_netlink.c:960:15
    FRRouting#4 0x5651c2 in netlink_route_change /home/sharpd/frr8/zebra/rt_netlink.c:1204:2
    FRRouting#5 0x54af15 in netlink_information_fetch /home/sharpd/frr8/zebra/kernel_netlink.c:407:10
    FRRouting#6 0x53e7a3 in netlink_parse_info /home/sharpd/frr8/zebra/kernel_netlink.c:1184:12
    FRRouting#7 0x548d46 in kernel_read /home/sharpd/frr8/zebra/kernel_netlink.c:501:2
    FRRouting#8 0x7feaeacc87f6 in thread_call /home/sharpd/frr8/lib/thread.c:2006:2
    FRRouting#9 0x7feaeab36503 in frr_run /home/sharpd/frr8/lib/libfrr.c:1198:3
    FRRouting#10 0x550d38 in main /home/sharpd/frr8/zebra/main.c:476:2
    FRRouting#11 0x7feaea492d09 in __libc_start_main csu/../csu/libc-start.c:308:16

SUMMARY: AddressSanitizer: 704 byte(s) leaked in 8 allocation(s).

Fix this!

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

zebra/rt_netlink.c

@@ -987,10 +987,12 @@ int netlink_route_change_read_unicast_internal(struct nlmsghdr *h,
  }
  }
  }
- if (nhe_id || ng)
+ if (nhe_id || ng) {
  dplane_rib_add_multipath(afi, SAFI_UNICAST, &p, &src_p,
  re, ng, startup, ctx);
- else {
+ if (ng)
+ nexthop_group_delete(&ng);
+ } else {
  /*
  * I really don't see how this is possible
  * but since we are testing for it let's