Fixed the memory leak. Refs #7506.

Memory leak is caused by mistakenly using ws.dataX(spectrum), which
causes a copy operatoin on vector, other than ws.readX(spectrum). As it
is not threading safe, the memory leak occurs.

Code/Mantid/Framework/Algorithms/src/FindPeaks.cpp

@@ -1166,7 +1166,7 @@ namespace Algorithms
                                         const int& i_centre, const int& i_min, const int& i_max,
                                         double& in_bg0, double& in_bg1, double& in_bg2)
   {
-    g_log.information() << "Fitting a peak assumed at " << input->dataX(spectrum)[i_centre]
+    g_log.information() << "Fitting a peak assumed at " << input->readX(spectrum)[i_centre]
                         << " (index = " << i_centre << ") by high-background approach. \n";
 
     // Check