CLEANUP: Apply ist.cocci

This cleans up ist handling.
diff --git a/src/hlua.c b/src/hlua.c
index a158c46..4bce23f 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -6315,8 +6315,7 @@
 
 			case HTX_BLK_DATA:
 				v = htx_get_blk_value(htx, blk);
-				v.ptr += offset;
-				v.len -= offset;
+				v = istadv(v, offset);
 				if (v.len > len)
 					v.len = len;
 
@@ -6417,7 +6416,7 @@
 		v.ptr += htxret.ret;
 		if (v.len > len)
 			v.len  = len;
-		blk = htx_replace_blk_value(htx, blk, v, ist2(NULL, 0));
+		blk = htx_replace_blk_value(htx, blk, v, IST_NULL);
 		len -= v.len;
 		ret += v.len;
 	}