Fix NPE when hitting Back button prematurely

- IMF RTC work item number: 15806
- Apache Cordova Jira issue: https://issues.apache.org/jira/browse/CB-2458
- Markmail or Google Group reference: n/a
- Description: patched CordovaWebView.java so that hitting the
  Back button when the app is still loading doesn't result in
  a NullPointerException.