[ftools] fix polygon from layer extent tool:

with the "Calculate extent for each feature separately" option ticked, it generates a python error.

Author: slarosa

python/plugins/fTools/tools/doGeometry.py

@@ -870,7 +870,8 @@ def layer_extent( self ):
 
     return True
 
-  def feature_extent( self, ):
+  def feature_extent( self ):
+    vprovider = self.vlayer.dataProvider()
     self.emit( SIGNAL( "runStatus( PyQt_PyObject )" ), 0 )
 
     fields = QgsFields()