Exception-proof the clearing of the username and password from a URL in case it doesn't understand the concept b=290829 r=biesi sr=dveditz a=asa

embedding/components/ui/helperAppDlg/nsHelperAppDlg.js

@@ -263,7 +263,9 @@ nsHelperAppDialog.prototype = {
 
          // Some URIs do not implement nsIURL, so we can't just QI.
          var url = this.mLauncher.source.clone();
-         url.userPass = "";
+         try {
+           url.userPass = "";
+         } catch (ex) {}
          var fname = "";
          this.mSourcePath = url.prePath;
          try {