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

src/manager.cpp

@@ -975,6 +975,11 @@ Manager::answerCall(const std::string& call_id)
         return false;
     }
 
+    if (call->getConnectionState() != Call::ConnectionState::RINGING) {
+        // The call is already answered
+        return true;
+    }
+
     // If ringing
     stopTone();