QObjectPrivate::Signal: initialize all members

A recent change in that area triggered Clang-SA to rescan the code and
complain about this pre-existing bug: If receiver == nullptr, then the
`previous` member was never initialized.

Fix by null'ing it using NSDMI. This is a trivial type, the compiler
will be able to avoid the redundant write.

Amends ab92b9e40025dcf08c14232de762a268201a78b4(!).

Pick-to: 6.6 6.5 6.2 5.15
Change-Id: Ideed71f0f36d5f896fb6a4614f233757c1371ee3
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 40714c1ddd886f6d0bde53e05f27d6b3e9e6649b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qobject_p_p.h

@@ -245,7 +245,7 @@ struct QObjectPrivate::Sender
             s = s->previous;
         }
     }
-    Sender *previous;
+    Sender *previous = nullptr;
     QObject *receiver;
     QObject *sender;
     int signal;