In commit cd7d6b08 ("Verify acceptor's mech in SPNEGO initiator") the
pointer sc->internal_mech became an alias into sc->mech_set->elements,
which should be considered constant for the duration of the SPNEGO
So don't free it.
This led to the obvious crashes in the allocator, and also to strange
behaviour with Firefox failing to fall back to alternative mechanisms
when it should have done.