Skip GC_DS_PER_OBJECT objects with negative descriptor in GC_mark_from

Skip GC_DS_PER_OBJECT objects with negative descriptor in GC_mark_from

Added a check in GC_mark_from() for GC_DS_PER_OBJECT objects with
negative descriptors to prevent mistaking the free list pointers in
free objects for being type descriptor pointers.  If the specified
descriptor offset was larger than the object size this could lead to
arbitrary data from allocated objects being misinterpreted as
descriptors and the process crashing.

* mark.c (GC_mark_from): In case of GC_DS_PER_OBJECT, skip objects
those descriptor is outside object.