Bugfixes for the Block-Level Sequence Producer API

[embg]

This PR fixes four bugs which we were very lucky to catch before the 1.5.4 release, mostly thanks to testing by @GarenJian-Intel (who caught three of the bugs).

Some of these bugs, it turns out, can be caught by our fuzzers with only a small modification to FUZZ_setRandomParameters(). I got fuzzing working on my machine, was able to reproduce bugs 1, 2, and 4, and will put that code up as a separate PR. I ran the fuzzers for a couple hours and they haven't found any additional bugs (we'll see if OSS-fuzz gets a different result). Will discuss with @terrelln whether it makes sense to modify our fuzzers so they can catch bug 3.

Bug 1

The external matchfinder API is not compatible with nbWorkers > 0 right now. This PR fails compression in that case, rather than failing an assert (debug builds) or crashing the application (release builds).

• Fix: see changes in ZSTD_CCtx_init_compressStream2()
• Test: see changes in zstreamtest.c
• Docs: updated zstd.h

Bug 2

Prior to this PR, some input sizes would lead to an incorrect estimate of memory required for the ZSTD_cwksp. This would cause compression to fail, since the workspace would run out of space for sub-allocations after making the initial shared allocation.

• Fix: see changes in ZSTD_estimateCCtxSize_usingCCtxParams_internal()
• Test: this will be covered by the fuzzers in my next PR
• Docs: n/a

Bug 3

The external matchfinder doesn't interact with older compression APIs, which means that this assert isn't always true. libzstd is behaving correctly, it's just the assert which is incorrect.

• Fix: delete the assert
• Test: see changes in zstreamtest.c
• Docs: updated zstd.h

Bug 4

When I was working on the fuzzers, I realized that I need a way to clear the external matchfinder. After discussing with @Cyan4973, I realized that there was a bug with calls of the form ZSTD_registerExternalMatchFinder(cctx, NULL, NULL), which should clear the matchfinder.

• Fix: modify ZSTD_registerExternalMatchFinder() such that it clears any previously registered external matchfinder when called with a NULL function pointer.
• Test: see changes in zstreamtest.c
• Docs: updated zstd.h