fix issues with decoding a pointer file from an empty buffer

lfs/pointer.go

@@ -64,11 +64,12 @@ func DecodePointer(reader io.Reader) (*Pointer, error) {
 func DecodeFrom(reader io.Reader) ([]byte, *Pointer, error) {
 	buf := make([]byte, 512)
 	written, err := reader.Read(buf)
+	output := buf[0:written]
+
 	if err != nil {
-		return buf, nil, err
+		return output, nil, err
 	}
 
-	output := buf[0:written]
 	p, err := decodeKV(bytes.TrimSpace(output))
 	return output, p, err
 }

lfs/pointer_test.go

@@ -3,6 +3,7 @@ package lfs
 import (
 	"bufio"
 	"bytes"
+	"io"
 	"strings"
 	"testing"
 
@@ -57,6 +58,21 @@ size 12345`
 	assertEqualWithExample(t, ex, int64(12345), p.Size)
 }
 
+func TestDecodeFromEmptyReader(t *testing.T) {
+	by, p, err := DecodeFrom(strings.NewReader(""))
+	if err != io.EOF {
+		t.Fatal("unexpected error: %v", err)
+	}
+
+	if p != nil {
+		t.Fatalf("Unexpected pointer: %v", p)
+	}
+
+	if string(by) != "" {
+		t.Fatalf("unexpected result: '%s'", string(by))
+	}
+}
+
 func TestDecodeInvalid(t *testing.T) {
 	examples := []string{
 		"invalid stuff",