Following up after <http://trac.webkit.org/changeset/162473>, we should break up
the assertion into two assertions as suggested by Darin Adler. Separating the single
assertion into two assertions makes it straightforward to determine the conjunct that
failed among other benefits.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::parseAttribute):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162503 268f45cc-cd09-0410-ab3c-d52691b4dbfc