Fix dangling reference in l2fib_scan(...)

Deleting a bihash kvp frees the bucket's backing storage when the
bucket reference count reaches zero. l2fib_scan MUST check for that
condition, and stop scanning the bucket if it occurs. One of the L2
FIB extended "make test" vectors caused this issue 100% of the time.

Change-Id: I250bcc4c1518e16042120fbc4032227a759a602e
Signed-off-by: Dave Barach <dave@barachs.net>
(cherry picked from commit 28374cada08df61180044e24cb758fa570e73c9d)

src/vnet/l2/l2_fib.c

@@ -1103,9 +1103,17 @@ l2fib_scan (vlib_main_t * vm, f64 start_time, u8 event_only)
 	      kv.key = key.raw;
 	      BV (clib_bihash_add_del) (&fm->mac_table, &kv, 0);
 	      learn_count--;
+	      /*
+	       * Note: we may have just freed the bucket's backing
+	       * storage, so check right here...
+	       */
+	      if (b->offset == 0)
+		goto doublebreak;
 	    }
 	  v++;
 	}
+    doublebreak:
+      ;
     }
 
   /* keep learn count consistent */