SkPngCodec: Do not return kInvalidConversion on corrupt png

In this case, the fuzzer thinks there is a bug because we are
returning kInvalidConversion for a corrupt png file.

Bug: skia:6550
Change-Id: I33f588442f5eaa8a4d642e9328750779f9a9ef5d
Reviewed-on: https://skia-review.googlesource.com/14324
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
1 file changed