QProcess/Windows: Fix handling of workingDirectory.

Store nativeWorkingDirectory in a variable on the stack to prevent
accessing the buffer of a temporary string.
Fix breakage introduced by 068baa9bb6d526cccc0c8bc7cdbb84bbdf137f95.

Change-Id: I8cf9ffe39c97afc9383c33cafaeffb1245b3e2b9
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/corelib/io/qprocess_win.cpp

@@ -498,11 +498,12 @@ void QProcessPrivate::startProcess()
                                  stdinChannel.pipe[0], stdoutChannel.pipe[1], stderrChannel.pipe[1]
     };
 
+    const QString nativeWorkingDirectory = QDir::toNativeSeparators(workingDirectory);
     QProcess::CreateProcessArguments cpargs = {
         0, (wchar_t*)args.utf16(),
         0, 0, TRUE, dwCreationFlags,
         environment.isEmpty() ? 0 : envlist.data(),
-        workingDirectory.isEmpty() ? 0 : (wchar_t*)QDir::toNativeSeparators(workingDirectory).utf16(),
+        nativeWorkingDirectory.isEmpty() ? Q_NULLPTR : (wchar_t*)nativeWorkingDirectory.utf16(),
         &startupInfo, pid
     };
     if (modifyCreateProcessArgs)