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/unix-misc.c b/src/vppinfra/unix-misc.c
index 5559a23..e73d01a 100644
--- a/src/vppinfra/unix-misc.c
+++ b/src/vppinfra/unix-misc.c
@@ -158,7 +158,7 @@
 
       if (bytes == 0)
 	{
-	  _vec_len (rv) = pos;
+	  vec_set_len (rv, pos);
 	  break;
 	}
       pos += bytes;