Megolm: don't dereference nullable object

It is possible for `room` to be null when passed to
MegolmEncryption.encryptMessage; we need to avoid dereferencing it. Instead,
make sure that the EncryptionAlgorithm knows about the roomId it is targeting,
and use that.

Replace the increasingly-long argument list on the EncryptionAlgorithm
constructor with a params list, and update DecryptionAlgorithm to match.

lib/crypto.js

@@ -625,7 +625,12 @@ Crypto.prototype.setRoomEncryption = function(roomId, config, roomMembers) {
     };
     this._sessionStore.storeEndToEndRoom(roomId, config);
 
-    var alg = new AlgClass(this._deviceId, this, this._olmDevice);
+    var alg = new AlgClass({
+        deviceId: this._deviceId,
+        crypto: this,
+        olmDevice: this._olmDevice,
+        roomId: roomId,
+    });
     this._roomAlgorithms[roomId] = alg;
     return alg.initRoomEncryption(roomMembers);
 };
@@ -719,7 +724,9 @@ Crypto.prototype.isRoomEncrypted = function(roomId) {
  * Encrypt an event according to the configuration of the room, if necessary.
  *
  * @param {module:models/event.MatrixEvent} event  event to be sent
- * @param {module:models/room} room  destination room
+ *
+ * @param {module:models/room?} room destination room. Null if the destination
+ *     is not a room we have seen over the sync pipe.
  */
 Crypto.prototype.encryptEventIfNeeded = function(event, room) {
     if (event.isEncrypted()) {
@@ -752,7 +759,7 @@ Crypto.prototype.encryptEventIfNeeded = function(event, room) {
     }
 
     var encryptedContent = alg.encryptMessage(
-         room, event.getType(), event.getContent()
+        room, event.getType(), event.getContent()
     );
     event.makeEncrypted("m.room.encrypted", encryptedContent);
 };
@@ -772,7 +779,9 @@ Crypto.prototype.decryptEvent = function(event) {
     if (!AlgClass) {
         throw new algorithms.DecryptionError("Unable to decrypt " + content.algorithm);
     }
-    var alg = new AlgClass(this._deviceId, this, this._olmDevice);
+    var alg = new AlgClass({
+        olmDevice: this._olmDevice,
+    });
     return alg.decryptEvent(event);
 };