Fix issue #51: Cache file written on first start, even if first update fails

Inadyn erroneously always created the cache file on first startup when
the cache file didn't exist yet, regardless if the update sent to the
DDNS server succeeded or not.

The cache file should only ever be written/created when the upstream
DDNS server has been successfully updated with our IP address.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>