archive/tar: Do not panic on Read if uninitialized

Calling tar.Reader.Read() used to work fine, but without this patch it panics.
Simply return EOF to indicate the tar.Reader.Next() needs to be called.

LGTM=iant, bradfitz
R=golang-codereviews, bradfitz, iant, mikioh.mikioh, dominik.honnef
CC=golang-codereviews
https://golang.org/cl/94530043

src/pkg/archive/tar/reader.go

@@ -724,6 +724,9 @@ func (tr *Reader) numBytes() int64 {
 // It returns 0, io.EOF when it reaches the end of that entry,
 // until Next is called to advance to the next entry.
 func (tr *Reader) Read(b []byte) (n int, err error) {
+	if tr.curr == nil {
+		return 0, io.EOF
+	}
 	n, err = tr.curr.Read(b)
 	if err != nil && err != io.EOF {
 		tr.err = err

src/pkg/archive/tar/reader_test.go

@@ -725,3 +725,19 @@ func TestReadGNUSparseMap1x0(t *testing.T) {
 		t.Errorf("Incorrect sparse map: got %v, wanted %v", sp, expected)
 	}
 }
+
+func TestUninitializedRead(t *testing.T) {
+	test := gnuTarTest
+	f, err := os.Open(test.file)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	defer f.Close()
+
+	tr := NewReader(f)
+	_, err = tr.Read([]byte{})
+	if err == nil || err != io.EOF {
+		t.Errorf("Unexpected error: %v, wanted %v", err, io.EOF)
+	}
+
+}