Before this fix, an assert could fail in PFS_lock::allocated_to_free(), during shutdown.
The assert itself is valid, and detects an anomaly caused by bug 56666.

While bug 56666 has no real consequences in production,
the failure caused by this new assert in the code is negatively
impacting the test suite with automated tests.

This fix is a work around only, that relaxes the integrity checks 
during the server shutdown.