Fix XIChangeHierarchy() integer underflow

CVE-2020-14346 / ZDI-CAN-11429

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative
Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>

Xi/xichangehierarchy.c

@@ -423,7 +423,7 @@ ProcXIChangeHierarchy(ClientPtr client)
     if (!stuff->num_changes)
         return rc;
 
-    len = ((size_t)stuff->length << 2) - sizeof(xXIChangeHierarchyReq);
+    len = ((size_t)client->req_len << 2) - sizeof(xXIChangeHierarchyReq);
 
     any = (xXIAnyHierarchyChangeInfo *) &stuff[1];
     while (stuff->num_changes--) {