[nocommit] Repro for zstd_decompress.c assert violation

[embg]

This is unrelated to magicless fuzzing -- the code in this PR uses the standard format decoder. I'm not sure yet if this is happening on valid frames or only invalid frames.

To run the fuzzer:

cd ~/repos/zstd/tests/fuzz
python3 ./fuzz.py build stream_decompress --enable-fuzzer --enable-asan --debug 1
python3 ./fuzz.py libfuzzer stream_decompress

The fuzzer should trigger the assert:

stream_decompress: /home/embg/repos/zstd/lib//decompress/zstd_decompress.c:1346: size_t ZSTD_decompressContinue(ZSTD_DCtx *, void *, size_t, const void *, size_t): Assertion `srcSize <= dctx->expected' failed.

Here is the failing assert: https://github.com/facebook/zstd/blob/0fcdc62c2d13f4d978ecba703c7d381ce97c97f1/lib/decompress/zstd_decompress.c#L1346

The assert was added in this PR: facebook#1947

[terrelln]

This call is UB when ZSTD_isError(ret1).

[embg]

OK, then there is no problem. Thanks for looking.