ICOImageDecoder: fix decodeFrameCount when there is no enough data to parse directory entries.
Initializing struct members helps exposing the issue - otherwise, it was reproducible always only with placement new when buffer was zeroed before it.

Re-enable DrMemory test for ICOImageDecoderTests.parseAndDecodeByteByByte.

BUG=621488

Review-Url: https://codereview.chromium.org/2081013002
Cr-Original-Commit-Position: refs/heads/master@{#402168}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4e621d9fc8a99383a04f5249f75f61a43cab2198