Fix handling of short streams

Before this change, on streams shorter than 6 bytes, we would present up
to 6 bytes of garbage at the start of the buffer.

With this change in place, we now present exactly what we read.

Introduced in e7ecaa2. Fixes #263.

m/zopen.go

@@ -103,14 +103,15 @@ func ZOpen(filename string) (io.ReadCloser, string, error) {
 func ZReader(input io.Reader) (io.Reader, error) {
 	// Read the first 6 bytes to determine the compression type
 	firstBytes := make([]byte, 6)
-	_, err := input.Read(firstBytes)
+	count, err := input.Read(firstBytes)
 	if err != nil {
 		if err == io.EOF {
 			// Stream was empty
 			return input, nil
 		}
 		return nil, fmt.Errorf("failed to read stream: %w", err)
 	}
+	firstBytes = firstBytes[:count]
 
 	// Reset input reader to start of stream
 	input = io.MultiReader(bytes.NewReader(firstBytes), input)

m/zopen_test.go

@@ -0,0 +1,36 @@
+package m
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+// Test that ZReader works with an empty stream
+func TestZReaderEmpty(t *testing.T) {
+	bytesReader := bytes.NewReader([]byte{})
+
+	zReader, err := ZReader(bytesReader)
+	assert.NilError(t, err)
+
+	all, err := io.ReadAll(zReader)
+	assert.NilError(t, err)
+
+	assert.Equal(t, 0, len(all))
+}
+
+// Test that ZReader works with a one-byte stream
+func TestZReaderOneByte(t *testing.T) {
+	bytesReader := bytes.NewReader([]byte{42})
+
+	zReader, err := ZReader(bytesReader)
+	assert.NilError(t, err)
+
+	all, err := io.ReadAll(zReader)
+	assert.NilError(t, err)
+
+	assert.Equal(t, 1, len(all))
+	assert.Equal(t, byte(42), all[0])
+}