Fix race in GC_print_all_errors regarding GC_leaked

* reclaim.c (GC_add_leaked): Remove FIXME.
* reclaim.c (GC_print_all_errors): Declare n_leaked, leaked[] local
variables initialized from GC_[n_]leaked while holding the allocation
lock (reset GC_n_leaked and GC_leaked[] as well); add GC_ASSERT for
n_leaked; use [n_]leaked while printing leaked objects count and
pointers.
* reclaim.c (GC_print_all_errors): Acquire lock to reset
printing_errors.