Fixing another clusterfuzz issue

Fixing another clusterfuzz issue

This was introduced by removing SkValidatingReadBuffer::readBitmap in https://codereview.chromium.org/295793002/

Since SkReadBuffer::skip wasn't virtual, it was using the unsafe SkReadBuffer::skip within SkReadBuffer::readBitmap rather than using SkValidatingReadBuffer::skip. I also removed direct uses of fReader within SkReadBuffer::readBitmap so that it can use the virtual readInt / readFixed functions that have a version in SkValidatingReadBuffer.

Also, I changed SkReadBuffer::readPoint so that it uses the virtual readScalar, that way, it becomes redundant with SkValidatingReadBuffer::readPoint, which can then be removed.

BUG=380723
R=reed@google.com, mtklein@google.com, sugoi@google.com

Author: sugoi@chromium.org

Review URL: https://codereview.chromium.org/317003003