QPostEventList: change pointer comparison to integer comparison

This is exactly the type of pointer arithmetics that GCC likes to
miscompile.

Just use offset >= size() (=subtract begin() from both sides).

Change-Id: Ifb13652d7b96bf4c06727d9c965516e95d16ab5c
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/thread/qthread_p.h

@@ -114,7 +114,7 @@ public:
         int priority = ev.priority;
         if (isEmpty() ||
             last().priority >= priority ||
-            begin() + insertionOffset >= end()) {
+            insertionOffset >= size()) {
             // optimization: we can simply append if the last event in
             // the queue has higher or equal priority
             append(ev);