- autohosts: fix crash on startup if filesystem watcher couldn't be initialized

Close #1814

Squashed commit of the following:

commit ba17a5b3c61a8a5282beb0cca470e2302e83c0d6
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Fri Jun 19 10:08:44 2020 +0300

    - autohosts: fix crash on startup if filesystem watcher couldn't be initialized

util/auto_hosts.go

@@ -76,6 +76,7 @@ func (a *AutoHosts) Start() {
 	go a.updateLoop()
 	a.updateChan <- true
 
+	if a.watcher != nil {
 		go a.watcherLoop()
 
 		err := a.watcher.Add(a.hostsFn)
@@ -89,13 +90,16 @@ func (a *AutoHosts) Start() {
 				log.Error("Error while initializing watcher for a directory %s: %s", dir, err)
 			}
 		}
+	}
 }
 
 // Close - close module
 func (a *AutoHosts) Close() {
 	a.updateChan <- false
 	close(a.updateChan)
+	if a.watcher != nil {
 		_ = a.watcher.Close()
+	}
 }
 
 // update table