[intfstat] fix --delete fail when counters dump does not exist (sonic…

…-net#752)

Signed-off-by: Mykola Faryma <mykolaf@mellanox.com>

scripts/intfstat

@@ -269,6 +269,10 @@ def main():
     cnstat_fqn_file = cnstat_dir + "/" + cnstat_file
 
     if delete_all_stats:
+        # There is nothing to delete
+        if not os.path.isdir(cnstat_dir):
+            sys.exit(0)
+
         for file in os.listdir(cnstat_dir):
             os.remove(cnstat_dir + "/" + file)