Check err param in filepath.WalkFunc (#6007) (#6009)

There was a missing error check in the file_integrity module's scanner that could result in a panic.

Fixes #6005

(cherry picked from commit e8db561)

CHANGELOG.asciidoc

@@ -32,6 +32,9 @@ https://github.com/elastic/beats/compare/v6.1.1...6.1[Check the HEAD diff]
 
 *Auditbeat*
 
+- Add an error check to the file integrity scanner to prevent a panic when
+  there is an error reading file info via lstat. {issue}6005[6005]
+
 *Filebeat*
 
 *Heartbeat*

auditbeat/module/audit/file/scanner.go

@@ -106,6 +106,14 @@ func (s *scanner) walkDir(dir string) error {
 	err := filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {
 		defer func() { startTime = time.Now() }()
 
+		if err != nil {
+			if !os.IsNotExist(err) {
+				logp.Warn("%v Scanner is skipping a path=%v because of an error: %v",
+					s.logPrefix, path, err)
+			}
+			return nil
+		}
+
 		event := s.newScanEvent(path, info, err)
 		event.rtt = time.Since(startTime)
 		select {