Don't try to remove a non-existent snapshot
https://bugs.webkit.org/show_bug.cgi?id=114226
<rdar://problem/13604480>

Reviewed by Darin Adler.

We don't need to trigger the timer to remove a snapshot
if there was never a snapshot displayed.

* html/HTMLPlugInImageElement.cpp:
(WebCore::HTMLPlugInImageElement::setDisplayState): Detect if we're moving from the DisplayingSnapshot state.
(WebCore::HTMLPlugInImageElement::removeSnapshotTimerFired): Guard against a missing renderer.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147977 268f45cc-cd09-0410-ab3c-d52691b4dbfc