QEventLoopLocker: defend against nullptr arguments

The class would previosuly produce a crash when the QEventLoop* or the
QThread* were nullptr or if no QCoreApplication existed. We want,
however, the out-of-line constructors of the class to be noexcept, and
for that, they should neither allocation nor have preconditions. The
former is for another patch; this patch deals with the latter.

[ChangeLog][QtCore][QEventLoopLocker] Is now a no-op on nullptr
QEventLoop*, QThread*, QCoreApplication::instance() (was: crash).

Task-number: QTBUG-114793
Pick-to: 6.6
Change-Id: I4246f74008df6ad7fcbfde56403397b065fbe861
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qeventloop.cpp

@@ -423,6 +423,8 @@ void QEventLoopLocker::visit(Func f) const
 {
     using Type = QEventLoopLockerPrivate::Type;
     const auto ptr = d_ptr->pointer();
+    if (!ptr)
+        return;
     switch (d_ptr->type()) {
     case Type::EventLoop:   return f(static_cast<QEventLoopPrivate *>(ptr));
     case Type::Thread:      return f(static_cast<QThreadPrivate *>(ptr));