insist SkPngEncoder's input is initialized

I'd rather catch uninitialized input here than
somewhere deep down in zlib or libpng.

Bug: chromium:1013368
Cq-Include-Trybots: skia.primary:Test-Debian9-Clang-GCE-CPU-AVX2-x86_64-Debug-All-MSAN
Change-Id: I37f5a66a8fc12c7e0ce6d3100de94021bb5343ef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/248551
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed