using SCM_SMOB_PREDICATE during GC

[Lars J. Aas]

I wrote something like this (guile 1.4) recently:

static SCM
my_smob_mark( SCM smob )
{
  SCM_ASSERT( SCM_SMOB_PREDICATE(smob_type, smob),
    smob, SCM_ARG1, "internal smob mark" );
  ...
}

I know it's unnecessary, but I wrote it just for the hell of it :)
Anyways, it always triggered the assert, so I'm led to believe that
there's a bit that's set during the mark sweep that is included when
extracting the smob-type from the smob (haven't investigated this).
If that is the case, it should probably be masked out.

  Lars J