Use nullptr instead of png_NULL in SkPNGImageEncoder

Also includes a few additional cleanups.

BUG=skia:

Change-Id: I50899bfef964a3391cc9ddf42c3c5a939e01ceae
Reviewed-on: https://skia-review.googlesource.com/6497
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
1 file changed