Checking for no IDBDatabase backend prior to use.

Speculative fix for a NULL ptr crash (crbug.com/362418) in
WebCore::IDBDatabase::transaction. In this case a IDBDatabase had been stopped
which results in the backend (blink::WebIDBDatabase) being deleted. If after
that the backend (m_backend) was used then a crash would ensue.

BUG=362418

Review URL: https://codereview.chromium.org/237393003

git-svn-id: svn://svn.chromium.org/blink/trunk@172438 bbb929c8-8fbe-4397-9dbb-9b2b20218538