archive/tar: add missing error checks

Check for errors when reading the headers following the pax headers.

Fixes #11169.

Change-Id: Ifec4a949ec8df8b49fa7cb7a67eb826fe2282ad8
Reviewed-on: https://go-review.googlesource.com/11031Reviewed-by: Russ Cox <rsc@golang.org>

src/archive/tar/reader.go

@@ -110,7 +110,13 @@ func (tr *Reader) Next() (*Header, error) {
 		// We actually read the whole file,
 		// but this skips alignment padding
 		tr.skipUnread()
+		if tr.err != nil {
+			return nil, tr.err
+		}
 		hdr = tr.readHeader()
+		if hdr == nil {
+			return nil, tr.err
+		}
 		mergePAX(hdr, headers)
 
 		// Check for a PAX format sparse file

src/archive/tar/reader_test.go

@@ -781,3 +781,18 @@ func TestIssue10968(t *testing.T) {
 		t.Fatalf("expected %q, got %q", io.ErrUnexpectedEOF, err)
 	}
 }
+
+// Do not panic if there are errors in header blocks after the pax header.
+// Issue 11169
+func TestIssue11169(t *testing.T) {
+	f, err := os.Open("testdata/issue11169.tar")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	r := NewReader(f)
+	_, err = r.Next()
+	if err == nil {
+		t.Fatal("Unexpected success")
+	}
+}