Protect against nil errors from inotify (elastic#7753) (elastic#7754) (…

…elastic#7756)

Users are experiencing occasional crashes in auditbeat that are caused
by a nil error being printed to the logs. Adding some defensive code to
prevent auditbeat from panicking.

Fixes elastic#7753

(cherry picked from commit 4843d6f)

CHANGELOG.asciidoc

@@ -33,6 +33,8 @@ https://github.com/elastic/beats/compare/v6.4.0...6.4[Check the HEAD diff]
 
 *Auditbeat*
 
+- Fixed a crash in the file_integrity module under Linux. {issue}7753[7753]
+
 *Filebeat*
 
 *Heartbeat*

auditbeat/module/file_integrity/eventreader_fsnotify.go

@@ -103,7 +103,11 @@ func (r *reader) consumeEvents(done <-chan struct{}) {
 
 			r.eventC <- e
 		case err := <-r.watcher.ErrorChannel():
-			r.log.Warnw("fsnotify watcher error", "error", err)
+			// a bug in fsnotify can cause spurious nil errors to be sent
+			// on the error channel.
+			if err != nil {
+				r.log.Warnw("fsnotify watcher error", "error", err)
+			}
 		}
 	}
 }