QSharedPointer: fix uninit'ed field 'destroyer'

The getAndRef(const QObject*) function failed to initialize
ExternalRefCountData's destroyer field: the Qt::Uninitialized ctor
doesn't init anything and there are no following assignments to the
field, either.

This probably a) caused some Coverty complaint (but I didn't check,
because the Coverity UI is so clumsy) and b) was harmless, seeing as
the -1 in strongref indicated that the destroyer is never to be fired.

Fix it nevertheless by initializing it to nullptr.

Amends the start of the public history.

Pick-to: 6.9 6.8 6.5
Change-Id: Ib76b4bc00b08289bb8d6d58096b43501b47814d1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 184d0bf2ec03dd3707b4b6326f6ba7e657afadc8)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qsharedpointer.cpp

@@ -1541,6 +1541,7 @@ QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::ge
     ExternalRefCountData *x = ::new ExternalRefCountData(Qt::Uninitialized);
     x->strongref.storeRelaxed(-1);
     x->weakref.storeRelaxed(2);  // the QWeakPointer that called us plus the QObject itself
+    x->destroyer = nullptr;
 
     ExternalRefCountData *ret;
     if (d->sharedRefcount.testAndSetOrdered(nullptr, x, ret)) {     // ought to be release+acquire; this is acq_rel+acquire