When receiving, add mock session if none exists

We need a session object in order to build a session from a
PreKeyWhisperMessage, so add an empty one when none exists on receiving
a message.

Warning: this will break right now if the session can not be constructed
from the received message.There will be an invalid session which will
break if we try to send using it.

src/main/java/eu/siacs/conversations/crypto/axolotl/AxolotlService.java

@@ -904,7 +904,7 @@ public class AxolotlService {
 			Log.d(Config.LOGTAG, "No axolotl session found while parsing received message " + message);
 			// TODO: handle this properly
 			session = new XmppAxolotlSession(axolotlStore, senderAddress);
-
+			sessions.put(senderAddress,session);
 		}
 
 		for(XmppAxolotlMessage.XmppAxolotlMessageHeader header : message.getHeaders()) {