Fix data race during apply_to_each_object(reset_back_edge)

Fix data race during apply_to_each_object(reset_back_edge)
(back-port of commit d7ea0b1 from 'master')

Issue #144 (bdwgc).

* backgraph.c [MAKE_BACK_GRAPH] (GC_print_back_graph_stats): Add
assertion that the allocation lock is held; store GC_deepest_obj to
obj local variable (while holding the lock); place UNLOCK/LOCK around
GC_print_heap_obj() call; pass obj to GC_print_heap_obj().
* finalize.c [MAKE_BACK_GRAPH] (GC_notify_or_invoke_finalizers): Remove
UNLOCK/LOCK around GC_print_back_graph_stats() call.