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/vppinfra/test_socket.c b/src/vppinfra/test_socket.c
index ea0ae65..3a0e6b2 100644
--- a/src/vppinfra/test_socket.c
+++ b/src/vppinfra/test_socket.c
@@ -99,7 +99,7 @@
 	break;
 
       if_verbose ("%v", s->rx_buffer);
-      _vec_len (s->rx_buffer) = 0;
+      vec_set_len (s->rx_buffer, 0);
     }
 
   error = clib_socket_close (s);