[processing] fixed fields calculator algorithm

It's not a complete fix, as it just comments out a line that was causing a 'underlying C/C++ object was deleted' error
Should revisit is later. For now, this allows the algorithm to run correctly and not crash qgis

Author: volaya

python/plugins/processing/algs/qgis/FieldsCalculator.py

@@ -101,8 +101,8 @@ def processAlgorithm(self, progress):
 
         da = QgsDistanceArea()
         da.setSourceCrs(layer.crs().srsid())
-        canvas = iface.mapCanvas()
-        da.setEllipsoidalMode(canvas.mapRenderer().hasCrsTransformEnabled())
+        #canvas = iface.mapCanvas()
+        #da.setEllipsoidalMode(iface.mapCanvas().mapRenderer().hasCrsTransformEnabled())
         da.setEllipsoid(QgsProject.instance().readEntry('Measure',
                                                         '/Ellipsoid',
                                                         GEO_NONE)[0])