journal: adapt for new improved LZ4_decompress_safe_partial()

journal: adapt for new improved LZ4_decompress_safe_partial()

With lz4 1.8.3, this function can now decompress partial results into a smaller
buffer. The release news don't say anything interesting, but the test case that
was previously failing now works OK.

Fixes #10259.

A test is added. It shows that with *older* lz4, a partial decompression can
occur with the returned size smaller then the requested number of bytes _and_
smaller then the size of the compressed data:

(lz4-libs-1.8.2-1.fc29.x86_64)
Compressed 4194304 → 16464
Decompressed → 4194304
Decompressed partial 12/4194304 → 4194304
Decompressed partial 1/1 → -2 (bad)
Decompressed partial 2/2 → -2 (bad)
Decompressed partial 3/3 → -2 (bad)
Decompressed partial 4/4 → -2 (bad)
Decompressed partial 5/5 → -2 (bad)
Decompressed partial 6/6 → 6 (good)
Decompressed partial 7/7 → 6 (good)
Decompressed partial 8/8 → 6 (good)
Decompressed partial 9/9 → 6 (good)
Decompressed partial 10/10 → 6 (good)
Decompressed partial 11/11 → 6 (good)
Decompressed partial 12/12 → 6 (good)
Decompressed partial 13/13 → 6 (good)
Decompressed partial 14/14 → 6 (good)
Decompressed partial 15/15 → 6 (good)
Decompressed partial 16/16 → 6 (good)
Decompressed partial 17/17 → 6 (good)
Decompressed partial 18/18 → -16459 (bad)

(lz4-libs-1.8.3-1.fc29.x86_64)
Compressed 4194304 → 16464
Decompressed → 4194304
Decompressed partial 12/4194304 → 12
Decompressed partial 1/1 → 1 (good)
Decompressed partial 2/2 → 2 (good)
Decompressed partial 3/3 → 3 (good)
Decompressed partial 4/4 → 4 (good)
...

If we got such a short "successful" decompression in decompress_startswith() as
implemented before this patch, we could be confused and return a false negative
result. But it turns out that this only occurs with small output buffer
sizes. We use greedy_realloc() to manager the buffer, so it is always at least
64 bytes. I couldn't hit a case where decompress_startswith() would actually
return a bogus result. But since the lack of proof is not conclusive, the code
for *older* lz4 is changed too, just to be safe. We cannot rule out that on a
different architecture or with some unlucky compressed string we could hit this
corner case.

The fallback code is guarded by a version check. The check uses a function not
the compile-time define, because there was no soversion bump in lz4 or new
symbols, and we could be compiled against a newer lz4 and linked at runtime
with an older one. (This happens routinely e.g. when somebody upgrades a subset
of distro packages.)