[rsyncable] Ensure ZSTD_compressBound() is respected

[rsyncable] Ensure ZSTD_compressBound() is respected

In degenerate cases `--rsyncable` could create very small blocks (1
byte). This causes the compressed output to be larger than
`ZSTD_compressBound()`. Fix the issue by ensuring that rsyncable mode
never outputs blocks smaller than 128 KB.

The minimum job size is 512 KB, so we shouldn't lose many
synchronization points from skipping any that cause blocks smaller than
128 KB. And even if we do, that is fine, because we'll find the next
one.

This fixes the `raw_dictionary_round_trip` oss-fuzz assert.

Credit to OSS-Fuzz