[backport] Fix bug where histogram can be assigned negative frequency…

… for a pixel range. Also fix potential memory leak as new histogram vector was assigned to band stats without clearing the old.

src/core/raster/qgsrasterlayer.cpp

@@ -2440,7 +2440,7 @@ void QgsRasterLayer::setDataProvider( QString const & provider,
     myRasterBandStats.bandName = mDataProvider->generateBandName( i );
     myRasterBandStats.bandNumber = i;
     myRasterBandStats.statsGathered = false;
-    myRasterBandStats.histogramVector = new QgsRasterBandStats::HistogramVector();
+    myRasterBandStats.histogramVector->clear();
     //Store the default color table
     //readColorTable( i, &myRasterBandStats.colorTable );
     QList<QgsColorRampShader::ColorRampItem> ct;

src/providers/gdal/qgsgdalprovider.cpp

@@ -1327,8 +1327,16 @@ void QgsGdalProvider::populateHistogram( int theBandNo,   QgsRasterBandStats & t
 
     for ( int myBin = 0; myBin < theBinCount; myBin++ )
     {
-      theBandStats.histogramVector->push_back( myHistogramArray[myBin] );
-      QgsDebugMsg( "Added " + QString::number( myHistogramArray[myBin] ) + " to histogram vector" );
+      if ( myHistogramArray[myBin] < 0 ) //can't have less than 0 pixels of any value
+      {
+        theBandStats.histogramVector->push_back( 0 );
+        QgsDebugMsg( "Added 0 to histogram vector as freq was negative!" );
+      }
+      else
+      {
+        theBandStats.histogramVector->push_back( myHistogramArray[myBin] );
+        QgsDebugMsg( "Added " + QString::number( myHistogramArray[myBin] ) + " to histogram vector" );
+      }
     }
 
   }