Fix a crash in HTMLFormElement::prepareForSubmission.

BUG=297478
TEST=automated with ASAN.

Review URL: https://chromiumcodereview.appspot.com/24910003

git-svn-id: svn://svn.chromium.org/blink/trunk@158428 bbb929c8-8fbe-4397-9dbb-9b2b20218538