call: ignore answerCall if already answered

Any client can try to answer a call multiple times. For example if dring
is running with the --auto-answer flag and the client set auto answer on
the daemon. This cause weird behavior like rotation not taken into account,
etc.

So, don't call answerCall if already answered.

Change-Id: Ibb967787aaf83e86a98fba7192810e86975304f1