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

@@ -1202,8 +1202,8 @@ 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.notice() << "Fitting a peak assumed at " << input->dataX(spectrum)[i_centre]
-                        << " (index = " << i_centre << ") by high-background approach. \n";
+    g_log.notice() << "Fitting a peak assumed at " << input->readX(spectrum)[i_centre]
+                   << " (index = " << i_centre << ") by high-background approach. \n";
 
     // Check
     if (i_min >= i_centre || i_max <= i_centre || i_min < 0)