[skjson] Catch end-of-stream for unbalanced scopes

The input is not guaranteed to contain well-formed scopes, so it's not
sufficient to check for end-of-stream only when popping the top-level
scope -- we have to check on every scope pop.

Bug: oss-fuzz:8898
Change-Id: I7399a8872187ec6714672cac2ff8fc7fbf3c2dfe
Reviewed-on: https://skia-review.googlesource.com/135059
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed