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/macros.c b/src/vppinfra/macros.c
index b8a8e17..27a92a8 100644
--- a/src/vppinfra/macros.c
+++ b/src/vppinfra/macros.c
@@ -175,7 +175,7 @@
 	      /* add results to answer */
 	      vec_append (rv, ts);
 	      /* Remove NULL termination or the results are sad */
-	      _vec_len (rv) = vec_len (rv) - 1;
+	      vec_set_len (rv, vec_len (rv) - 1);
 	      vec_free (ts);
 	    }
 	  else
@@ -183,8 +183,7 @@
 	      if (complain)
 		clib_warning ("Undefined Variable Reference: %s\n", varname);
 	      vec_append (rv, format (0, "UNSET "));
-	      _vec_len (rv) = vec_len (rv) - 1;
-
+	      vec_set_len (rv, vec_len (rv) - 1);
 	    }
 	  vec_free (varname);
 	}