vtkChartBox: replaced a retained, returned pointer with a stored
value.

vtkChartXY: Added check for range[0] == range[1] and avoid division by zero.

vtkFunctionParser: Added check for division of a vector by 0.0.

vtkUnstructuredGridQuadricDecimation: first time through loop,
lasterror is 0.0. Avoid division by zero by checking for 0.0.