Fix null-ptr access crash on ScriptLoader.

Adding a null check where crash reports tell.
The crash seems to happen when the frame of a document is removed
in certain timing, but I couldn't reproduce for that.

TEST=none
BUG=267176
R=inferno,abarth

Review URL: https://chromiumcodereview.appspot.com/22527002

git-svn-id: svn://svn.chromium.org/blink/trunk@155665 bbb929c8-8fbe-4397-9dbb-9b2b20218538