Do not leave blocking section twice

Test by running ocsigen with ocaml-ssl from master: as soon
as it accepts an ssl connection it'll just hang/misbehave because
Ocaml's internal lock will be in an inconsistent state.

This bug was likely introduced by 13f860b8165566bcf2b5c25ba5118eb330667625,
so it doesn't affect the release ocaml-ssl-4.6, only master.