File transfer plugin - upload to invalid server can cause app to crash on iOS

This was discovered when investigating that the file transfer plugin automatic tests would consistently cause mobile spec to crash on iOS. The specific offending test is "filetransfer.spec.25 should handle unknown host".

Investigation reveals that when an invalid host is specified as the upload target, on iOS, the upload will fail in the background. Part of the plugin's native iOS error handling is to delete the "target" file. When the upload target does not include a path (e.g. the test uses "http://foobar.apache.org"), an error will occur trying to delete a nil path.