s2: Fix incorrect seek for io.SeekEnd (#575)

Offset sign was flipped

Fixes #574

s2/cmd/s2d/main.go

@@ -278,7 +278,7 @@ Options:`)
 					rs, err := r.ReadSeeker(tailBytes > 0, nil)
 					exitErr(err)
 					if tailBytes > 0 {
-						_, err = rs.Seek(int64(tailBytes), io.SeekEnd)
+						_, err = rs.Seek(-int64(tailBytes), io.SeekEnd)
 					} else {
 						_, err = rs.Seek(int64(offset), io.SeekStart)
 					}

s2/decode.go

@@ -699,8 +699,16 @@ func (r *ReadSeeker) Seek(offset int64, whence int) (int64, error) {
 	case io.SeekCurrent:
 		offset += r.blockStart + int64(r.i)
 	case io.SeekEnd:
-		offset = -offset
+		if offset > 0 {
+			return 0, errors.New("seek after end of file")
+		}
+		offset = r.index.TotalUncompressed + offset
+	}
+
+	if offset < 0 {
+		return 0, errors.New("seek before start of file")
 	}
+
 	c, u, err := r.index.Find(offset)
 	if err != nil {
 		return r.blockStart + int64(r.i), err
@@ -712,10 +720,6 @@ func (r *ReadSeeker) Seek(offset int64, whence int) (int64, error) {
 		return 0, err
 	}
 
-	if offset < 0 {
-		offset = r.index.TotalUncompressed + offset
-	}
-
 	r.i = r.j // Remove rest of current block.
 	if u < offset {
 		// Forward inside block

s2/encode_test.go

@@ -333,7 +333,7 @@ func TestIndex(t *testing.T) {
 					case io.SeekCurrent:
 						toSkip = wantOffset - int64(len(input)/2)
 					case io.SeekEnd:
-						toSkip = int64(len(input)) - wantOffset
+						toSkip = -(int64(len(input)) - wantOffset)
 					}
 					gotOffset, err := rs.Seek(toSkip, i)
 					if gotOffset != wantOffset {