zstd: Don't allocate dataStorage when using byteBuf (#741)

Credit to @greatroar for finding this.

Alternative to #721

zstd/blockdec.go

@@ -192,16 +192,14 @@ func (b *blockDec) reset(br byteBuffer, windowSize uint64) error {
 	}
 
 	// Read block data.
-	if cap(b.dataStorage) < cSize {
+	if _, ok := br.(*byteBuf); !ok && cap(b.dataStorage) < cSize {
+		// byteBuf doesn't need a destination buffer.
 		if b.lowMem || cSize > maxCompressedBlockSize {
 			b.dataStorage = make([]byte, 0, cSize+compressedBlockOverAlloc)
 		} else {
 			b.dataStorage = make([]byte, 0, maxCompressedBlockSizeAlloc)
 		}
 	}
-	if cap(b.dst) <= maxSize {
-		b.dst = make([]byte, 0, maxSize+1)
-	}
 	b.data, err = br.readBig(cSize, b.dataStorage)
 	if err != nil {
 		if debugDecoder {
@@ -210,6 +208,9 @@ func (b *blockDec) reset(br byteBuffer, windowSize uint64) error {
 		}
 		return err
 	}
+	if cap(b.dst) <= maxSize {
+		b.dst = make([]byte, 0, maxSize+1)
+	}
 	return nil
 }