lib/model: Fix race in GetIgnores (fixes syncthing#4300)

In addition this function returned an error when .stignore file was not
present, which is perfectly valid. Also removed inconsistent nil check in
ignores.go (only relevant for tests) and adjusted walk.go to do what it says
in comments (check if Matcher is nil) to prevent nil deref in tests.

Originally reported in:
https://forum.syncthing.net/t/reason-for-panic-maybe-too-little-ram/10346

Regression from syncthing#3996

GitHub-Pull-Request: syncthing#4301

lib/model/model.go

@@ -1245,16 +1245,15 @@ func (m *Model) ConnectedTo(deviceID protocol.DeviceID) bool {
 
 func (m *Model) GetIgnores(folder string) ([]string, []string, error) {
 	m.fmut.RLock()
+	defer m.fmut.RUnlock()
+
 	cfg, ok := m.folderCfgs[folder]
-	m.fmut.RUnlock()
 	if ok {
 		if !cfg.HasMarker() {
 			return nil, nil, fmt.Errorf("Folder %s stopped", folder)
 		}
 
-		m.fmut.RLock()
 		ignores := m.folderIgnores[folder]
-		m.fmut.RUnlock()
 
 		return ignores.Lines(), ignores.Patterns(), nil
 	}