Fix filling of incomplete images in SkSampledCodec

The bug occurs when filling after an incomplete sampled/subset
incremental decode.

I'm also adding many truncated test pngs.

BUG=skia:5769
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2343153003

Review-Url: https://codereview.chromium.org/2343153003
2 files changed