txtar/archive: ignore invalid file separators

In case the file separator line does not define a filename, it should be
ignored.

Updates golang/go#47193

Change-Id: I3a0fee584c0f6b2b41814e79d20884d6468a3b76
Reviewed-on: https://go-review.googlesource.com/c/tools/+/336932
Run-TryBot: Jay Conrod <jayconrod@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

txtar/archive.go

@@ -121,7 +121,7 @@ func isMarker(data []byte) (name string, after []byte) {
 	if i := bytes.IndexByte(data, '\n'); i >= 0 {
 		data, after = data[:i], data[i+1:]
 	}
-	if !bytes.HasSuffix(data, markerEnd) {
+	if !(bytes.HasSuffix(data, markerEnd) && len(data) >= len(marker)+len(markerEnd)) {
 		return "", nil
 	}
 	return strings.TrimSpace(string(data[len(marker) : len(data)-len(markerEnd)])), after

txtar/archive_test.go

@@ -29,7 +29,10 @@ More file 1 text.
 File 2 text.
 -- empty --
 -- noNL --
-hello world`,
+hello world
+-- empty filename line --
+some content
+-- --`,
 			parsed: &Archive{
 				Comment: []byte("comment1\ncomment2\n"),
 				Files: []File{
@@ -37,6 +40,7 @@ hello world`,
 					{"file 2", []byte("File 2 text.\n")},
 					{"empty", []byte{}},
 					{"noNL", []byte("hello world\n")},
+					{"empty filename line", []byte("some content\n-- --\n")},
 				},
 			},
 		},