ADD_GC_CALLBACK() at the end of do_gc() leaks.
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=72
Reported by @grubba
dmalloc + debug(4):
Exited normally, counting bytes.
Freeing storage for small block still in use 144dd4c at
/home/grubba/src/Pike7.1/src/callback.c:38.
<<<=- No known references to this block -=>>>
*** /home/grubba/src/Pike7.1/src/gc.c:2191 (1 times)
*** /home/grubba/src/Pike7.1/src/callback.c:38 (1 times)
==LEAK==: (144dd4c) 16 bytes
**Location: 144dd4c Type: unknown Refs: 1431655765