Fix segfault in vtkMatplotlibMathTextUtilities destructor

Several of the ParaView Python tests fail with a segfault in the
destructor of the vtkMatplotlibMathTextUtilities when it calls
Py_Finalize(). This changes the destructor to only call Py_Finalize()
if vtkMatplotlibMathTextUtilities itself called Py_Initialize().

Change-Id: I7fab0273a65c6e6784f1acf4266dfc53594d5fca
Merge topic 'fix-matplotlib-math-text-segfault' into master

169c9bf5 Fix segfault in vtkMatplotlibMathTextUtilities destructor