Ensure that mNotificationManager is not null when updating transfer progress

This might happen, if the FileUploader service is being destroyed,
so mNotificationManager is set to null.

The applied fix has been used in a couple of other places in this file
as well, so it is at least consistent.

Signed-off-by: Torsten Grote <t@grobox.de>

src/main/java/com/owncloud/android/files/services/FileUploader.java

@@ -715,6 +715,9 @@ public class FileUploader extends Service
             String fileName = filePath.substring(filePath.lastIndexOf(FileUtils.PATH_SEPARATOR) + 1);
             String text = String.format(getString(R.string.uploader_upload_in_progress_content), percent, fileName);
             mNotificationBuilder.setContentText(text);
+            if (mNotificationManager == null) {
+                mNotificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
+            }
             mNotificationManager.notify(R.string.uploader_upload_in_progress_ticker, mNotificationBuilder.build());
         }
         mLastPercent = percent;