Consistency check in count_memory() is broken.

Imported from https://youtrack.roxen.com/issue/PIKE-226

Reported by @grubba

Doing tests in testsuite (12090 tests, pid 28821)
Testing scoped variables 17:16...
Assertion failed: !(m->flags & MC_FLAG_INTERNAL), file gc.c, line 5901, function f_count_memory

No result from subprocess (died of signal SIGABRT)
Subresult: 1 tests, 1 failed, 0 skipped

The test that causes the assertion failure seems to be

test_any([[
  /* don't save parent */
  class X (void|object a) {};
  X o = X (X (X()));
  o->a->a->a = o->a;
  return count_memory_stats (2, o);
]], [["i:3,c:2,e:0,v:7,r:4,rnd:2"]])