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/mhash.c b/src/vppinfra/mhash.c
index c556312..f0f1aa4 100644
--- a/src/vppinfra/mhash.c
+++ b/src/vppinfra/mhash.c
@@ -295,7 +295,7 @@
 	{
 	  i = h->key_vector_free_indices[l - 1];
 	  k = vec_elt_at_index (h->key_vector_or_heap, i);
-	  _vec_len (h->key_vector_free_indices) = l - 1;
+	  vec_set_len (h->key_vector_free_indices, l - 1);
 	}
       else
 	{
@@ -332,10 +332,10 @@
 	  if (key_alloc_from_free_list)
 	    {
 	      h->key_vector_free_indices[l] = i;
-	      _vec_len (h->key_vector_free_indices) = l + 1;
+	      vec_set_len (h->key_vector_free_indices, l + 1);
 	    }
 	  else
-	    _vec_len (h->key_vector_or_heap) -= h->n_key_bytes;
+	    vec_dec_len (h->key_vector_or_heap, h->n_key_bytes);
 	}
     }