QProcess/Win: Avoid reentering the code in _q_processDied()

In case drainOutputPipes() makes a recursion, setting 'dying' to true
should protect the code from reentering. But, the next call to
QProcessPrivate::cleanup() resets this variable, which allows a
secondary pass. So, we should postpone setting 'dying' to false until
a new process session is started.

Fixes: QTBUG-33731
Change-Id: I269ad3b8defa32aa714ea13f8803a07259f475dc
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess.cpp

@@ -894,7 +894,6 @@ void QProcessPrivate::cleanup()
 #endif
     pid = 0;
     sequenceNumber = 0;
-    dying = false;
 
     if (stdoutChannel.notifier) {
         delete stdoutChannel.notifier;
@@ -2148,6 +2147,7 @@ void QProcessPrivate::start(QIODevice::OpenMode mode)
     stderrChannel.closed = false;
 
     exitCode = 0;
+    dying = false;
     exitStatus = QProcess::NormalExit;
     processError = QProcess::UnknownError;
     errorString.clear();