QThread: initialize interruptionRequested atomic

Coverity complains correctly that the variable is not initialized.
std::atomic's default constructor doesn't do it, and we only do it
in QThread::start and QThread::finished. So initialize it using NSMDI.

Pick-to: 6.7 6.5
Coverity-Id: 466429
Change-Id: I36d4624d46718f50e10bec17ef5e437c60541fa9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit d1f6d2258809ed7e2d7aa50de3b6d6f472cf1bd8)
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/thread/qthread_p.h

@@ -187,7 +187,7 @@ public:
     bool running;
     bool finished;
     bool isInFinish; //when in QThreadPrivate::finish
-    std::atomic<bool> interruptionRequested;
+    std::atomic<bool> interruptionRequested = false;
 
     bool exited;
     int returnCode;