Zstd workspace poisoning doesn't unpoison before memory is freed

[terrelln]

This causes issues when using a custom allocator that doesn't actually free() memory.

1. Use a custom allocator
2. Use the allocator for compression
3. Compression poisons part of the memory
4. Free that memory
5. Re-use that custom allocator for decompression
6. Decompression will run into use-after-poison issues

Workaround: define ZSTD_ASAN_DONT_POISON_WORKSPACE

[terrelln]

CC @felixhandte