Fix memory leak in QtConcurrent::run when called with a NULL QThreadPool

QThreadPool automatically deletes the runnable after it finishes running
the task. In case QThreadPool is nullptr, we should delete the runnable
manually. This amends 87b93c29be02f0a7ff9424b5e2b6431e20bd4c40.

Pick-to: 6.3 6.2 5.15
Change-Id: Id7e4ed3d4d6de05990edf62e4099852983debc64
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/concurrent/qtconcurrentrunbase.h

@@ -99,6 +99,7 @@ public:
         } else {
             promise.reportCanceled();
             promise.reportFinished();
+            delete this;
         }
         return theFuture;
     }

tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp

@@ -66,6 +66,7 @@ private slots:
     void customPromise();
     void nonDefaultConstructibleValue();
     void nullThreadPool();
+    void nullThreadPoolNoLeak();
 };
 
 void light()
@@ -1589,5 +1590,27 @@ void tst_QtConcurrentRun::nullThreadPool()
     QVERIFY(!isInvoked);
 }
 
+struct LifetimeChecker
+{
+    LifetimeChecker() { ++count; }
+    LifetimeChecker(const LifetimeChecker &) { ++count; }
+    ~LifetimeChecker() { --count; }
+
+    void operator()() { }
+
+    static std::atomic<int> count;
+};
+std::atomic<int> LifetimeChecker::count = 0;
+
+void tst_QtConcurrentRun::nullThreadPoolNoLeak()
+{
+    {
+        QThreadPool *pool = nullptr;
+        auto future = run(pool, LifetimeChecker());
+        future.waitForFinished();
+    }
+    QCOMPARE(LifetimeChecker::count, 0);
+}
+
 QTEST_MAIN(tst_QtConcurrentRun)
 #include "tst_qtconcurrentrun.moc"