_memory_usage() returns negative num_objects value

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2625

Reported by Martin Nilsson, IDA nilsson@pike.ida.liu.se

Pike v7.3 release 12 running Hilfe v2.0 (Incremental Pike Frontend)
> _memory_usage();
Result: ([ /* 18 elements */
          "array_bytes":17804,
          "callable_bytes":8200,
          "callback_bytes":2052,
          "frame_bytes":10756,
          "mapping_bytes":52295,
          "multiset_bytes":304,
          "num_arrays":159,
          "num_callables":220,
          "num_callbacks":2,
          "num_frames":9,
          "num_mappings":45,
          "num_multisets":19,
          "num_objects":-56,
          "num_programs":104,
          "num_strings":2922,
          "object_bytes":185076,
          "program_bytes":563152,
          "string_bytes":109240
        ])