Prevent use of NULL IndexedDB backend database.

When an IndexedDB connection (IDBDatabase) is closed it closes it's backend
db (WebIDBDatabase) and set's it's ptr to NULL. If a transaction is used
after that, it may attempt to use that NULL backend db ptr. This change adds
a NULL ptr check before use to avoid a crash.

Speculative fix for crbug.com/361278

BUG=361278

Review URL: https://codereview.chromium.org/230503006

git-svn-id: svn://svn.chromium.org/blink/trunk@171174 bbb929c8-8fbe-4397-9dbb-9b2b20218538