bond: Fix bug that writes to freed memory

pr_op->pr_rule is pointing to memory in bond->hash. It shouldn't be written
if bond->hash is already freed.

This bug is reported by running kernel path testsuite under valgrind:
Invalid write of size 8
   at 0x413D16: update_recirc_rules__ (bond.c:392)
   by 0x414CA0: bond_unref (bond.c:290)
   by 0x427E3C: bundle_destroy (ofproto-dpif.c:3002)
   by 0x429EF4: bundle_set (ofproto-dpif.c:3023)
   by 0x40858B: port_destroy (bridge.c:4087)
   by 0x40BD04: bridge_destroy (bridge.c:3266)
   by 0x410528: bridge_exit (bridge.c:506)
   by 0x4072EE: main (ovs-vswitchd.c:135)
 Address 0xb5a85f0 is 5,360 bytes inside a block of size 12,288 free'd
   at 0x4C2EDEB: free (/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x414C8D: bond_unref (bond.c:288)
   by 0x427E3C: bundle_destroy (ofproto-dpif.c:3002)
   by 0x429EF4: bundle_set (ofproto-dpif.c:3023)
   by 0x40858B: port_destroy (bridge.c:4087)
   by 0x40BD04: bridge_destroy (bridge.c:3266)
   by 0x410528: bridge_exit (bridge.c:506)
   by 0x4072EE: main (ovs-vswitchd.c:135)
 Block was alloc'd at
   at 0x4C2DB8F: malloc (/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x516C04: xmalloc (util.c:120)
   by 0x414FD1: bond_entry_reset (bond.c:1651)
   by 0x414FD1: bond_reconfigure (bond.c:470)
   by 0x41507D: bond_create (bond.c:245)
   by 0x429D5D: bundle_set (ofproto-dpif.c:3194)
   by 0x408AC8: port_configure (bridge.c:1052)
   by 0x40CD87: bridge_reconfigure (bridge.c:682)
   by 0x410775: bridge_run (bridge.c:2998)
   by 0x407244: main (ovs-vswitchd.c:119)

Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>

ofproto/bond.c

@@ -389,7 +389,9 @@ update_recirc_rules__(struct bond *bond)
             }
 
             hmap_remove(&bond->pr_rule_ops, &pr_op->hmap_node);
-            *pr_op->pr_rule = NULL;
+            if (bond->hash) {
+                *pr_op->pr_rule = NULL;
+            }
             free(pr_op);
             break;
         }