Fix panic on add_docker_metadata close (elastic#21882)

If the processor was not properly initialized, for example because it
couldn't access the docker socket, then the watcher will be nil. Avoid
trying to stop the watcher in that case.

(cherry picked from commit 80b8f53)

libbeat/processors/add_docker_metadata/add_docker_metadata.go

@@ -213,7 +213,10 @@ func (d *addDockerMetadata) Close() error {
 	if d.cgroups != nil {
 		d.cgroups.StopJanitor()
 	}
-	d.watcher.Stop()
+	// Watcher can be nil if processor failed on creation
+	if d.watcher != nil {
+		d.watcher.Stop()
+	}
 	err := processors.Close(d.sourceProcessor)
 	if err != nil {
 		return errors.Wrap(err, "closing source processor of add_docker_metadata")