PaintOpReader: Harden PaintImage deserialization

Add missing validity check after `Read`

Bug: 327698060
Change-Id: I0aa5120296009998af3235a01304a1f597a82a33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5337387


Commit-Queue: Khushal Sagar <khushalsagar@chromium.org>
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267636}