Fix memory leak in vtkPNGWriter when OutOfDiskSpaceError

Change-Id: Ib5f0adc17054a3a4c490bd45c980b7c8d2de932a
Initialize row_pointers

Change-Id: Ie12cce088a6ed29f8c73a39587610db2cdf2e3ee
Merge topic 'vtkPNGWriter-memory-leak' into master

d5e12400 Initialize row_pointers
adf24a53 Fix memory leak in vtkPNGWriter when OutOfDiskSpaceError