Bug 1157946 - Early return if aResponse.sources is undefined;r=fitzgen

This happens quite often during the damp devtools talos test, when
the toolbox is closed immediately after a page is reloaded

browser/devtools/debugger/debugger-controller.js

@@ -1222,7 +1222,7 @@ SourceScripts.prototype = {
    * Callback for the debugger's active thread getSources() method.
    */
   _onSourcesAdded: function(aResponse) {
-    if (aResponse.error) {
+    if (aResponse.error || !aResponse.sources) {
       let msg = "Error getting sources: " + aResponse.message;
       Cu.reportError(msg);
       dumpn(msg);