Check the user is set before filtering transfers

This solves issue #11. The error is thrown on load because the user is only set after the dropdown value has changed. This doesn't attempt to filter the transfers if the user hasn't been set.

LFS171x/sawtooth-material/sawtooth-tuna/client/src/app.js

@@ -49,9 +49,11 @@ app.refresh = function () {
       }
     })
 
-    // Populate transfer list for selected user
-    transfers.filter(transfer => transfer.owner === this.user.public)
-      .forEach(transfer => addAction('#transferList', transfer.asset, 'Accept'))
+    // Populate transfer list for selected user (if there is a selected user)
+    if(this.user) {
+      transfers.filter(transfer => transfer.owner === this.user.public)
+        .forEach(transfer => addAction('#transferList', transfer.asset, 'Accept'))
+    }
 
     // Populate transfer select with both local and blockchain keys
     let publicKeys = this.keys.map(pair => pair.public)