[zstdmt] Fix determinism issue with rsyncable mode

The problem occurs in this scenario:
1. We find a synchronization point.
2. We attmept to create the job.
3. We fail because the job table is full: `mtctx->nextJobID > mtctx->doneJobID + mtctx->jobIDMask`.
4. We call `ZSTDMT_compressStream_generic` again.
5. We forget that we're at a sync point already, and we continue looking
   for the next sync point.

This fix is to detect if we're currently paused at a sync point, and if
we are then don't load any more input.

Caught by zstreamtest. I modified it to make the bug occur more often
(~1/100K -> ~1/200) and verified that it is fixed after. I then ran a
few hundred thousand unmodified zstreamtest iterations to verify.

lib/compress/zstdmt_compress.c

@@ -1687,6 +1687,16 @@ findSynchronizationPoint(ZSTDMT_CCtx const* mtctx, ZSTD_inBuffer const input)
         pos = 0;
         prev = (BYTE const*)mtctx->inBuff.buffer.start + mtctx->inBuff.filled - RSYNC_LENGTH;
         hash = ZSTD_rollingHash_compute(prev, RSYNC_LENGTH);
+        if ((hash & hitMask) == hitMask) {
+            /* We're already at a sync point so don't load any more until
+             * we're able to flush this sync point.
+             * This likely happened because the job table was full so we
+             * couldn't add our job.
+             */
+            syncPoint.toLoad = 0;
+            syncPoint.flush = 1;
+            return syncPoint;
+        }
     } else {
         /* We don't have enough bytes buffered to initialize the hash, but
          * we know we have at least RSYNC_LENGTH bytes total.