nsIInputStream can exist and then may later go away when the stream becomes unreadable (closed/errored). Since FetchStreamReader is created only when nsIInputStream initially did not exist, the current assertion may fail.
This patch now goes LockStream way if FetchStreamReader doesn't exist, regardless of whether nsIInputStream exists or not.
Differential Revision: https://phabricator.services.mozilla.com/D194413
292605a3e7