zebra: Skip route table lookup if zvrf is NULL

A crash was observed in 8.4.4 zebra when FRR processes were being shutting down by
automated script:

  Thread 1 (LWP 16051):
  #0  0x00007f63f449bb8f in raise () from /lib64/libpthread.so.0
  FRRouting#1  0x00007f63f5c68300 in core_handler (signo=11, siginfo=0x7ffec6322cb0, context=<optimized out>) at lib/sigevent.c:261
  FRRouting#2  <signal handler called>
  FRRouting#3  zebra_router_get_table (zvrf=zvrf@entry=0x0, tableid=tableid@entry=254, afi=afi@entry=AFI_IP, safi=safi@entry=SAFI_UNICAST) at /usr/include/bits/string_fortified.h:71
  FRRouting#4  0x0000560d74192e6d in zebra_vrf_get_table_with_table_id (afi=AFI_IP, safi=SAFI_UNICAST, vrf_id=<optimized out>, table_id=254) at zebra/zebra_vrf.c:335
  FRRouting#5  0x0000560d74186d20 in process_subq_early_route_add (ere=<optimized out>) at zebra/zebra_rib.c:2649
  FRRouting#6  process_subq_early_route (lnode=0x560d7783c5f0) at zebra/zebra_rib.c:3127
  FRRouting#7  process_subq (qindex=META_QUEUE_EARLY_ROUTE, subq=0x560d75cb1e40) at zebra/zebra_rib.c:3150
  FRRouting#8  meta_queue_process (dummy=<optimized out>, data=0x560d75cba680) at zebra/zebra_rib.c:3202
  FRRouting#9  0x00007f63f5c84550 in work_queue_run (thread=0x7ffec63233d0) at lib/workqueue.c:285
  FRRouting#10 0x00007f63f5c7a4c1 in thread_call (thread=thread@entry=0x7ffec63233d0) at lib/thread.c:2008
  FRRouting#11 0x00007f63f5c32088 in frr_run (master=0x560d75acbf50) at lib/libfrr.c:1216
  FRRouting#12 0x0000560d7411c8f7 in main (argc=<optimized out>, argv=0x7ffec63237a8) at zebra/main.c:499

Below is analysis for the sequence of events which led to zebra crash:

 - configs including VRF configs were deleted in zebra
 - Some route messages for the deleted VRF were still in the zebra metaq waiting
   to be processed
 - when the route message was dequeued for processing, the VRF was already deleted
 - lookup of zvrf failed for the route vrf_id in route-entry, but the NULL return
   was not checked, resulted in SIGSEGV crash when it was dereferenced later

Signed-off-by: Jenny Yuan <jyuan@paloaltonetworks.com>

zebra/zebra_vrf.c

@@ -316,6 +316,13 @@ struct route_table *zebra_vrf_get_table_with_table_id(afi_t afi, safi_t safi,
 	struct other_route_table *otable;
 	struct route_table *table;
 
+	/* VRF config could be deleted while some route messages for the VRF were
+	 * still in zebra metaq pending processing. Skip vrf table lookup for the
+	 * route if zvrf for vrf_id is already NULL
+	 */
+	if (zvrf == NULL)
+		return NULL;
+
 	table = zebra_vrf_lookup_table_with_table_id(afi, safi, vrf_id,
 						     table_id);