Update the MOZ_ASSERT in nsFrameLoader::GetBrowsingContext
With the change in bug 1882670, nsFrameLoader::GetBrowsingContext could
be called when we've destroyed the nsFrameLoader.
Differential Revision: https://phabricator.services.mozilla.com/D222153
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1918505
gecko-commit: 29d6035fe8eee3eb31f4d79f06a1394e1aafe094
gecko-reviewers: emilio