Fix unprotected access to QDBusPendingCallPrivate::pending.

In QDBusConnectionPrivate::waitForFinished() pcall->pending was used
after the protection by pcall->mutex was released. A simultaneous
call to QDBusConnectionPrivate::processFinishedCall() was able
to reset pcall->pending to null before it was used for the
q_dbus_pending_call_block(pcall->pending) call.

Fixed by releasing (and setting to 0) of pcall->pending in
processFinishedCall() only in case no one is waiting yet, otherwise
release pcall->pending by the first thread waiting in waitForFinished().

There is still a race condition about deleting QDBusPendingCallPrivate
(too early) which will be fixed in the next two commits.

Task-number: QTBUG-27809
Change-Id: I040173810ad90653fe1bd1915f22d8dd70d47d8c
Remove QDBusPendingCallPrivate::autoDelete logic.

First step to fix race condition about deleting QDBusPendingCallPrivate.

In a multithreaded application on a slow/single core cpu the following
race (and segmentation fault) can occur:

First thread A is running:
A:  QDBusPendingReply<> reply = pi->asyncCallWithArgumentList(method, argumentList);

Then when the dbus answer arrives thread B will call:

B:  QDBusConnectionPrivate::processFinishedCall()
B:      ...
B:      locker.unlock()

and runs until here, go on with thread A:

A:  reply.waitForFinished();
A:    QDBusPendingCallPrivate::waitForFinished()
A:    {
A:        QMutexLocker locker(&mutex);
A:        if (replyMessage.type() != QDBusMessage::InvalidMessage)
A:            return;

which returns immediately (mutex acquired, replyMessage alread set), now
reply goes out of scope (destructor called) and QDBusPendingCall::d's
destructor of type QExplicitlySharedDataPointer<QDBusPendingCallPrivate>
deletes the reference counted object QDBusPendingCallPrivate.

Now thread B continues, still in processFinishedCall()

B:      if (call->watcherHelper)
B:          call->watcherHelper->emitSignals(msg, call->sentMessage);
B:
B:      if (msg.type() == QDBusMessage::ErrorMessage)
B:          emit connection->callWithCallbackFailed(QDBusError(msg),
B:              call->sentMessage);

accessing alread deleted object QDBusPendingCallPrivate via call->...

Fixed QDBusPendingCallPrivate deletion by proper reference counting
will be done in the next commit.

Task-number: QTBUG-27809
Change-Id: I15b3f0242471b62eaafadc763fb6a33339ff2fe1
Change QDBusPendingCallPrivate to full reference counting for deletion.

Fixes race between QDBusConnectionPrivate::processFinishedCall()
releasing the mutex before emitting signals (using various members of
QDBusPendingCallPrivate) and deletion of the QDBusPendingCallPrivate
object through QDBusPendingCall::d's destructor (a member of type
QExplicitlySharedDataPointer<QDBusPendingCallPrivate>) leeds to
segmentation fault with CrashTest example on slow/single core
arm cpu).

Task-number: QTBUG-27809
Change-Id: I3590d74d1cfa5816ede764b50b83a7008ec780ff