QDuplicateTracker: fix the static buffer size calculation

Instead of just sizeof(T), we, of course, also need to take the
support structure into account, to wit: the bucket list and, in the
node, the next pointer and the stored hash value.

Change-Id: I8227a95c49e316aacf3d4efd8f6170ea3bea1cf0
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 3c88e12beb22d8ea11b8a7006a71ba9773c6d183)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qduplicatetracker_p.h

@@ -73,7 +73,13 @@ class QDuplicateTracker {
         }
     };
 
-    char buffer[Prealloc * sizeof(T)];
+    struct node_guesstimate { void *next; size_t hash; T value; };
+    static constexpr size_t bufferSize(size_t N) {
+        return N * sizeof(void*) // bucket list
+                + N * sizeof(node_guesstimate); // nodes
+    }
+
+    char buffer[bufferSize(Prealloc)];
     std::pmr::monotonic_buffer_resource res{buffer, sizeof buffer};
     std::pmr::unordered_set<T, QHasher<T>> set{&res};
 #else