prevents exception when logFormatted is called from javascript code injected from content-scripts

sourceLink is null when all the callers in the stack dump have system urls (e.g. chrome:// and resource://):

https://github.com/firebug/firebug/blob/master/extension/content/firebug/console/consoleExposed.js#L381

and it raise an exception.

Raising an exception currently prevents javascript code injected from a content-script to use the Firebug console object to print logs, and content-scripts are commonly used by Firefox Add-ons based on Addon SDK to interact with a content page (as in tildeio/ember-extension#94)