gopls/internal/util/frob: Decode: improve panic on empty

Also, add justification for fall through.

Updates golang/go#71244

Change-Id: I781d015a9d4659815588e95dea92eb350388b925
Reviewed-on: https://go-review.googlesource.com/c/tools/+/642435
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/cache/check.go

@@ -1290,6 +1290,7 @@ func (s *Snapshot) typerefData(ctx context.Context, id PackageID, imports map[Im
 		return data, nil
 	} else if err != filecache.ErrNotFound {
 		bug.Reportf("internal error reading typerefs data: %v", err)
+		// Unexpected error: treat as cache miss, and fall through.
 	}
 
 	pgfs, err := s.view.parseCache.parseFiles(ctx, token.NewFileSet(), parsego.Full&^parser.ParseComments, true, cgfs...)

gopls/internal/util/frob/frob.go

@@ -244,8 +244,8 @@ func (fr *frob) Decode(data []byte, ptr any) {
 		panic(fmt.Sprintf("got %v, want %v", rv.Type(), fr.t))
 	}
 	rd := &reader{data}
-	if string(rd.bytes(4)) != magic {
-		panic("not a frob-encoded message")
+	if len(data) < len(magic) || string(rd.bytes(len(magic))) != magic {
+		panic("not a frob-encoded message") // (likely an empty message)
 	}
 	fr.decode(rd, rv)
 	if len(rd.data) > 0 {