Examples: fix UB calls into QElapsedTimer

Calling restart() without start() is illegal, so don't do that; use
start() instead.

Pick-to: 6.8 6.5
Change-Id: I2ff5151f588cb926ccecc3d9997615e63f36ee24
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 029407117f0584aea7722ff7702c9f4b83fa9013)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/corelib/threads/mandelbrot/renderthread.cpp

@@ -81,7 +81,7 @@ void RenderThread::run()
             constexpr int Limit = 4;
             bool allBlack = true;
 
-            timer.restart();
+            timer.start();
 
             for (int y = -halfHeight; y < halfHeight; ++y) {
                 if (restart)

examples/network/network-chat/connection.cpp

@@ -237,7 +237,7 @@ void Connection::processData()
         writer.endMap();
         break;
     case Pong:
-        pongTime.restart();
+        pongTime.start();
         break;
     default:
         break;