[GTK] Test /webkit2/WebKitUserContentManager/injected-script fails
https://bugs.webkit.org/show_bug.cgi?id=135696

Reviewed by Sergio Villar Senin.

Don't assume the JavaScript result is always a valid pointer. In case of JavaScript
exception (that happens when testing the script hasn't been injected) the JavaScript
result is NULL.

* TestWebKitAPI/Tests/WebKit2Gtk/TestWebKitUserContentManager.cpp:
(isScriptInjectedForURLAtPath):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172206 268f45cc-cd09-0410-ab3c-d52691b4dbfc