let's try to fix nullptr + k

Apply -fsanitize-recover=pointer-overflow to third_party code only.

I'm trying to keep Skia behaving the same, avoiding illegal nullptr+k:
    - Add null check in SkJSON fast string path.
    - Add null check (first alloc) and some comments to SkArenaAlloc.
    - March an int index instead dst pointer in SkBase64.

Bug: skia:9731
Change-Id: I646635558ea63ded846b746f2a1f0b4f1e1eacff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268109
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
5 files changed