tst_qtconcurrentrun: fix UB (uninit'ed values)

While doubleFunction's body is empty, it takes its argument by value,
which means copying. Copying an uninitialized double is a gray zone
(if you follow the partially-formed paradigma, it's UB; though the std
may allow it some types, most notably std::byte and uchar; probably
not double, though). Converting an uninitialized int into double is
most certainly UB.

Fix by initializing both d and i.

Found by GCC 11's -Wmaybe-uninitialized.

Pick-to: 6.5 6.2 5.15
Change-Id: I103fb72bf4b8792a292346007f498dc6349e9c68
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp

@@ -666,10 +666,10 @@ void tst_QtConcurrentRun::implicitConvertibleTypes()
 {
     QThreadPool pool;
 
-    double d;
+    double d = 0.0;
     run(doubleFunction, d).waitForFinished();
     run(&pool, doubleFunction, d).waitForFinished();
-    int i;
+    int i = 0;
     run(doubleFunction, d).waitForFinished();
     run(&pool, doubleFunction, d).waitForFinished();
     run(doubleFunction, i).waitForFinished();