Fix crash in cleanupAfterDeletion when destination is null

Blink r154347 avoided adding a placeholder in certain cases
when editing, but it incorrectly didn't check for a valid
destination in cleanupAfterDeletion, resulting in a null
pointer dereference and crash.

BUG=154347

Review URL: https://chromiumcodereview.appspot.com/23272003

git-svn-id: svn://svn.chromium.org/blink/trunk@156249 bbb929c8-8fbe-4397-9dbb-9b2b20218538