[gdal] Fix raster block output when block extent is outside of valid area

This was causing problems in 3D view when raster was used as DEM... terrain
tiles were generated also where they should not appear and contained random
heights that caused failures of camera center adjustment to terrain. In the areas
completely outside of raster's extent the block was being returned with some
values uninitialized (instead of having correctly set no-data values)

(cherry picked from commit afca644)

Author: wonder-sk

src/providers/gdal/qgsgdalprovider.cpp

@@ -640,6 +640,13 @@ QgsRasterBlock *QgsGdalProvider::block( int bandNo, const QgsRectangle &extent,
     return block;
   }
 
+  if ( !mExtent.intersects( extent ) )
+  {
+    // the requested extent is completely outside of the raster's extent - nothing to do
+    block->setIsNoData();
+    return block;
+  }
+
   if ( !mExtent.contains( extent ) )
   {
     QRect subRect = QgsRasterBlock::subRect( extent, width, height, mExtent );