Two fixes for module bug when interleaving threads

[Philipp Stephani]

Hi,

I just pushed two fixes for the same bug that appears when
interleaving module calls in separate threads:
cdc632fbe6e149318147a98cccf1b7af191f2ce8 for the release branch and
23974cfa48b9245658667eff81d132b3aecd2618 for master. The intention was
that a simpler and more localized fix should go to the release branch,
the "proper" fix to master. Now I've realized that arguably the
simpler fix is overall better: it's more localized, touches fewer
moving pieces, and is probably not significantly slower (though I
haven't benchmarked it). Any opinions? If people agree, we can
overwrite the fix on master with the fix on the release branch.

Thanks,
Philipp