Report 0 rowsDecoded for no rows (SkPngCodec interlaced)

incrementalDecode is supposed to report the number of rows decoded. It
failed to if none were decoded (for an interlaced png). Fix the bug and
add a test.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2402063002

Review-Url: https://codereview.chromium.org/2402063002
2 files changed