huff0: Prevent single 4X blocks exceeding 16 bits (#507)

When used with max sized blocks, make sure the "jump table" can contain each output size.

zstd cannot trigger this, since max block size is half.

huff0/compress.go

@@ -2,6 +2,7 @@ package huff0
 
 import (
 	"fmt"
+	"math"
 	"runtime"
 	"sync"
 )
@@ -289,6 +290,10 @@ func (s *Scratch) compress4X(src []byte) ([]byte, error) {
 		if err != nil {
 			return nil, err
 		}
+		if len(s.Out)-idx > math.MaxUint16 {
+			// We cannot store the size in the jump table
+			return nil, ErrIncompressible
+		}
 		// Write compressed length as little endian before block.
 		if i < 3 {
 			// Last length is not written.
@@ -332,6 +337,10 @@ func (s *Scratch) compress4Xp(src []byte) ([]byte, error) {
 			return nil, errs[i]
 		}
 		o := s.tmpOut[i]
+		if len(o) > math.MaxUint16 {
+			// We cannot store the size in the jump table
+			return nil, ErrIncompressible
+		}
 		// Write compressed length as little endian before block.
 		if i < 3 {
 			// Last length is not written.