Fix a bug in the unification.

The `skip` counter, introduced as an optmization to avoid unnecessary
assertions, was decremented correctly but not incremented/restored well.
This patch fix this bug.
[auto] Merge from Compiler

    • #8d2070d — Fix a bug in the unification. (Ivan Enderlin, 2014-11-11)

---

Merge branch 'master' of /var/lib/git/repositories/Library/Compiler