Fix "thread.{de,}allocatedp" mallctl.

For the non-TLS case (as on OS X), if the "thread.{de,}allocatedp"
mallctl was called before any allocation occurred for that thread, the
TSD was still NULL, thus putting the application at risk of
dereferencing NULL.  Fix this by refactoring the initialization code,
and making it part of the conditional logic for all per thread
allocation counter accesses.