[zstdmt] Fix jobsize bugs (#1205)

[zstdmt] Fix jobsize bugs (#1205)

[zstdmt] Fix jobsize bugs

* `ZSTDMT_serialState_reset()` should use `targetSectionSize`, not `jobSize` when sizing the seqstore.
  Add an assert that checks that we sized the seqstore using the right job size.
* `ZSTDMT_compressionJob()` should check if `rawSeqStore.seq == NULL`.
* `ZSTDMT_initCStream_internal()` should not adjust `mtctx->params.jobSize` (clamping to MIN/MAX is okay).