Fix a crash when fetching stations & offline
Fix a crash when fetching stations & offline

Scenario:
 - device is offline
 - station JSON is being loaded from a remote file
 - load fails
 - application crashes

 Fix:
  - change the check in StationsViewController to verify that the
    station count is > 0 before trying to update the table view.
    Pull-to-refresh is still enabled, so the app remains responsive
    until the network connection is reestablished and the JSON can be
    downloaded.

tuanan94/FRadio-ios,tuanan94/FRadio-ios,tuanan94/FRadio-ios