vppinfra: make _vec_len() read-only

Use of _vec_len() to set vector length breaks address sanitizer.
Users should use vec_set_len(), vec_inc_len(), vec_dec_len () instead.

Type: improvement
Change-Id: I441ae948771eb21c23a61f3ff9163bdad74a2cb8
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vnet/ip/ip_api.c b/src/vnet/ip/ip_api.c
index 3ff3a6c..ae4ee0c 100644
--- a/src/vnet/ip/ip_api.c
+++ b/src/vnet/ip/ip_api.c
@@ -1237,7 +1237,7 @@
 
   /* Gather interfaces. */
   sorted_sis = vec_new (vnet_sw_interface_t, pool_elts (im->sw_interfaces));
-  _vec_len (sorted_sis) = 0;
+  vec_set_len (sorted_sis, 0);
   /* *INDENT-OFF* */
   pool_foreach (si, im->sw_interfaces)
    {