Avoid integer overflow in SkIcoCodec

Avoid integer overflow in SkIcoCodec

Definitely good to avoid overflow here.

FWIW, this looks to be harmless for Android's current use.
They will just fail later on when trying to allocate the
bitmap.

BUG=skia:5857

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3527

Change-Id: Ia1fb7d864d21ecdb127a1dd1a72cab8375cb43fb
Reviewed-on: https://skia-review.googlesource.com/3527
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>