No error thrown when there is garbage data past end-of-frame

[embg]

On 0.17.0, the following example doesn't throw an error:

>>> zstandard.decompress(zstandard.compress(b"foo") + b"garbage")
b'foo'

Perhaps debatable, but I think an error should always be thrown if there is data past the last valid frame in a stream. Especially after fixing #59, I don't think it makes sense to silently ignore the garbage past end-of-frame. Users should be notified if they are constructing invalid streams.

Thanks @thatch for identifying this issue.

[indygreg]

Ignoring junk data after a valid zstd frame is intended for the decompressobj() API, as that is what the stdlib API does.

But this report is for the ZstdDecompressor.decompress() API, which is different.

I agree the one-shot API should probably reject garbage past end-of-frame. The API docs even say this:

The input bytes are expected to contain a full Zstandard frame. ... If the input does not contain a full frame, an exception will be raised.

So I consider this a bug.

Hyrum's Law says someone is likely relying on the buggy behavior. So we may want to provide an argument to maintain the existing behavior, just in case.