Create group2sessions.sessionIDs if it doesn't exist yet.

src/node/db/SessionManager.js

@@ -139,7 +139,7 @@ exports.createSession = function(groupID, authorID, validUntil, callback)
         if(ERR(err, callback)) return;
         
         //the entry doesn't exist so far, let's create it
-        if(group2sessions == null)
+        if(group2sessions == null || group2sessions.sessionIDs == null)
         {
           group2sessions = {sessionIDs : {}};
         }