sstable: fix row writer mangling of cache block

Fix `RawRowWriter.addDataBlock` to make a copy of the block data when
it's not compressed (similar to the columnar writer), because in that
case the writing out can mangle the buffer.

The sstable copier (during Download) uses `addDataBlock` with a buffer
from the cache, which we are not allowed to modify.

Informs cockroachdb/cockroach#138662

sstable/rowblk_writer.go

@@ -1942,8 +1942,21 @@ func (w *RawRowWriter) copyDataBlocks(
 
 // addDataBlock implements RawWriter.
 func (w *RawRowWriter) addDataBlock(b, sep []byte, bhp block.HandleWithProperties) error {
+	blockBuf := &w.dataBlockBuf.blockBuf
+	pb := block.CompressAndChecksum(
+		&blockBuf.compressedBuf,
+		b,
+		w.layout.compression,
+		&blockBuf.checksummer,
+	)
+	if !pb.IsCompressed() {
+		// If the block isn't compressed, pb's underlying data points
+		// directly b. Clone it before writing it, as writing can mangle the buffer.
+		pb = pb.Clone()
+	}
+
 	// layout.WriteDataBlock keeps layout.offset up-to-date for us.
-	bh, err := w.layout.WriteDataBlock(b, &w.dataBlockBuf.blockBuf)
+	bh, err := w.layout.writePrecompressedBlock(pb)
 	if err != nil {
 		return err
 	}