Avoid validating freshly mapped memory.

Move validation of supposedly zeroed pages from chunk_alloc() to
chunk_recycle().  There is little point to validating newly mapped
memory returned by chunk_alloc_mmap(), and memory that comes from sbrk()
is explicitly zeroed, so there is little risk to assuming that
chunk_alloc_dss() actually does the zeroing properly.

This relaxation of validation can make a big difference to application
startup time and overall system usage on platforms that use jemalloc as
the system allocator (namely FreeBSD).

Submitted by Ian Lepore <ian@FreeBSD.org>.