[lib] Add ZSTD_d_stableOutBuffer + fix single-pass mode for empty frames

[terrelln]

This flag allows users to skip the output buffer allocation + memcpy if they guarantee that the ZSTD_outBuffer* passed to ZSTD_decompressStream() is large enough to fit the entire frame, and is never modified by the user.

Also fixes a bug where empty frames weren't eligible for single-pass mode. It caused a small allocation (4 bytes), and is a little bit slower.

Adds tests for single-pass and stable output buffer modes.

Fuzz tests ZSTD_d_stableOutput to ensure it never crashes.

Fixes #2093

[Cyan4973]

Looks good to me.
It's pretty well done, respecting project's coding style and responsibility layers, complete with a test set.

I'm just curious how many users would be interested by this new capability ?
Does it go beyond #2093 in your opinion ?

[terrelln]

I'm just curious how many users would be interested by this new capability ?
Does it go beyond #2093 in your opinion ?

Yes, I expect this to be a reasonably common scenario, as far as streaming goes. We could use this in Managed Compression and folly::io::Codec. They can both take folly::IOBuf as an input, which is a chain of input buffers, and both generally write to a single output buffer.

Most of the time the IOBuf's aren't chained, but when they are we either have to copy the input into a single buffer (Managed Compression does this), or use streaming (folly does this). Both would be served by this API. Managed Compression would save an allocation, and might save copies if there are blocks that are entirely contained in one buffer. folly would save the allocation of the output buffer, and the extra memcpy.

[terrelln]

Fixed a memory leak in zstreamtest