Replace unnecessary null-checks with an assert in MainResourceLoader::continueAfterNavigationPolicy.
https://bugs.webkit.org/show_bug.cgi?id=106476

Reviewed by Alexey Proskuryakov.

* loader/MainResourceLoader.cpp:
(WebCore::MainResourceLoader::continueAfterNavigationPolicy): Null-checks for resourceLoader were
    added in an abundance of caution in r139150. Given that we know the load hasn't been cancelled,
    resourceLoader can only be null if continueAfterNavigationPolicy() has already been called
    before when m_substituteData is valid. continueAfterNavigationPolicy() is only called for redirects,
    and SubstituteData doesn't support redirects, so it is correct to assert that resourceLoader is non-null.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139350 268f45cc-cd09-0410-ab3c-d52691b4dbfc