zebra: Cleanup use after free in shutdown

zebra: Cleanup use after free in shutdown

On shutdown a use after free was being seen of a route table.
Basically the pointer was kept around and resent for cleanup.
Probably something needs to be unwound to make this better
in the future.  Just cleaning up the use after free.

./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-=================================================================
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929:==911929==ERROR: AddressSanitizer: heap-use-after-free on address 0x606000127a00 at pc 0x7fb9ad546f5b bp 0x7ffc3cff0330 sp 0x7ffc3
cff0328
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-READ of size 8 at 0x606000127a00 thread T0
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #0 0x7fb9ad546f5a in route_table_free /home/sharpd/frr8/lib/table.c:103:13
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #1 0x7fb9ad546f04 in route_table_finish /home/sharpd/frr8/lib/table.c:61:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #2 0x6b94ba in zebra_ns_disable_internal /home/sharpd/frr8/zebra/zebra_ns.c:141:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #3 0x6b9158 in zebra_ns_disabled /home/sharpd/frr8/zebra/zebra_ns.c:116:9
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #4 0x7fb9ad43f0f5 in ns_disable_internal /home/sharpd/frr8/lib/netns_linux.c:273:4
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #5 0x7fb9ad43e634 in ns_disable /home/sharpd/frr8/lib/netns_linux.c:368:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #6 0x7fb9ad43e251 in ns_delete /home/sharpd/frr8/lib/netns_linux.c:330:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #7 0x7fb9ad43fbb3 in ns_terminate /home/sharpd/frr8/lib/netns_linux.c:524:3
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #8 0x54f8de in zebra_finalize /home/sharpd/frr8/zebra/main.c:232:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #9 0x7fb9ad5655e6 in thread_call /home/sharpd/frr8/lib/thread.c:2006:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #10 0x7fb9ad3d3343 in frr_run /home/sharpd/frr8/lib/libfrr.c:1198:3
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #11 0x550b48 in main /home/sharpd/frr8/zebra/main.c:476:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #12 0x7fb9acd30d09 in __libc_start_main csu/../csu/libc-start.c:308:16
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #13 0x443549 in _start (/usr/lib/frr/zebra+0x443549)
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-0x606000127a00 is located 0 bytes inside of 56-byte region [0x606000127a00,0x606000127a38)
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-freed by thread T0 here:
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #0 0x4bd33d in free (/usr/lib/frr/zebra+0x4bd33d)
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #1 0x7fb9ad42cc80 in qfree /home/sharpd/frr8/lib/memory.c:141:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #2 0x7fb9ad547305 in route_table_free /home/sharpd/frr8/lib/table.c:141:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #3 0x7fb9ad546f04 in route_table_finish /home/sharpd/frr8/lib/table.c:61:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #4 0x6b94ba in zebra_ns_disable_internal /home/sharpd/frr8/zebra/zebra_ns.c:141:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #5 0x6b9692 in zebra_ns_early_shutdown /home/sharpd/frr8/zebra/zebra_ns.c:164:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #6 0x7fb9ad43f228 in ns_walk_func /home/sharpd/frr8/lib/netns_linux.c:386:9
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #7 0x55014f in sigint /home/sharpd/frr8/zebra/main.c:194:2
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #8 0x7fb9ad50db99 in frr_sigevent_process /home/sharpd/frr8/lib/sigevent.c:130:6
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #9 0x7fb9ad560d07 in thread_fetch /home/sharpd/frr8/lib/thread.c:1775:4
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #10 0x7fb9ad3d332d in frr_run /home/sharpd/frr8/lib/libfrr.c:1197:9
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #11 0x550b48 in main /home/sharpd/frr8/zebra/main.c:476:2
--
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-    #7 0x7fb9acd30d09 in __libc_start_main csu/../csu/libc-start.c:308:16
./bfd_vrf_topo1.test_bfd_vrf_topo1/r2.zebra.asan.911929-

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