The crash of https://bugzilla.mozilla.org/show_bug.cgi?id=1835468 can be
reproduced by HTTP redirect in mochitest-plain.
When the script is being processed, sometimes the preloaded request is
still being loaded (for example, the script is being redirected to
another place). So we use the IsModulleFetched() check before calling
RemoveFetchedModule.
Differential Revision: https://phabricator.services.mozilla.com/D182271
