Fix avoid crash when measuring empty geometry

Author: manisandro

src/core/qgsdistancearea.cpp

@@ -889,6 +889,11 @@ void QgsDistanceArea::computeAreaInit()
 
 double QgsDistanceArea::computePolygonArea( const QList<QgsPoint>& points ) const
 {
+  if ( points.isEmpty() )
+  {
+    return 0;
+  }
+
   double x1, y1, x2, y2, dx, dy;
   double Qbar1, Qbar2;
   double area;