Fix a chunk recycling bug.

Fix a chunk recycling bug that could cause the allocator to lose track
of whether a chunk was zeroed.  On FreeBSD, NetBSD, and OS X, it could
cause corruption if allocating via sbrk(2) (unlikely unless running with
the "dss:primary" option specified).  This was completely harmless on
Linux unless using mlockall(2) (and unlikely even then, unless the
--disable-munmap configure option or the "dss:primary" option was
specified).  This regression was introduced in 3.1.0 by the
mlockall(2)/madvise(2) interaction fix.