V8LazyEventListener::prepareListenerObject should have a check for a null context

It looks possible that V8LazyEventListener::prepareListenerObject is called with a null ExecutionContext. (I don't know a concrete scenario but it will happen if a parent document requests an event handler of an object in a child document that is already disposed.)

Just like V8LazyEventListener::prepareListenerObject has a check for a null v8::Context, we should have a check for a null ExecutionContext.

BUG=418340
TEST=None. I manually verified that this CL fixes the bug 418340 but failed in minimizing the test case. Cluster Fuzz didn't provide a minimized test.

Review URL: https://codereview.chromium.org/622483002

git-svn-id: svn://svn.chromium.org/blink/trunk@183046 bbb929c8-8fbe-4397-9dbb-9b2b20218538