Fix a thread cache stats merging bug.

When a thread cache flushes objects to their arenas due to an abundance
of cached objects, it merges the allocation request count for the
associated size class, and increments a flush counter.  If none of the
flushed objects came from the thread's assigned arena, then the merging
wouldn't happen (though the counter would typically eventually be
merged), nor would the flush counter be incremented (a hard bug).  Fix
this via extra conditional code just after the flush loop.