KURL not handling NULL m_string members properly.

Due to some recent refactorings, its possible to get a NULL
m_string inside of KURL given some bad UFT-8 input. This
corrects the checks in several places where we would formerly
segv.

BUG=275868
R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24095009

git-svn-id: svn://svn.chromium.org/blink/trunk@157922 bbb929c8-8fbe-4397-9dbb-9b2b20218538