Fuzzer: Gracefully handle hashes that cannot be serialized

Add unserializehash fuzzer.

Unlike the straight unserialize fuzzer, this runs only on HashContexts,
and it does an update and finalize on the contexts it creates.

Co-authored-by

Add unserializehash fuzzer.

Unlike the straight unserialize fuzzer, this runs only on HashContexts,
and it does an update and finalize on the contexts it creates.

Co-authored-by: Nikita Popov <nikic@php.net>

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