encryption: Fix olm decryption if we already have a session.

matrix/encryption.py

@@ -222,11 +222,14 @@ class Olm():
                         continue
 
                 plaintext = session.decrypt(message)
-                break
+                return plaintext
             except OlmSessionError:
                 pass
 
-        session = self._create_session(sender, sender_key, message)
+        try:
+            session = self._create_session(sender, sender_key, message)
+        except OlmSessionError:
+            return None
 
         try:
             plaintext = session.decrypt(message)

matrix/events.py

@@ -475,6 +475,9 @@ class MatrixSyncEvent(MatrixEvent):
         olm = server.olm
         plaintext = olm.decrypt(sender, sender_key, message)
 
+        if not plaintext:
+            return None
+
         # TODO check sender key
         parsed_plaintext = json.loads(plaintext, encoding='utf-8')
         decrypted_sender = parsed_plaintext["sender"]