Fix NULL dereference by count_memory() when running --with-dmalloc.
Imported from https://youtrack.roxen.com/issue/PIKE-24
Reported by @grubba
Blocking PIKE-20
Don't attempt to get gc markers when the gc isn't active.
Imported from https://youtrack.roxen.com/issue/PIKE-24
Reported by @grubba
Blocking PIKE-20
Don't attempt to get gc markers when the gc isn't active.