Fix a xallocx(..., MALLOCX_ZERO) bug.

Fix xallocx(..., MALLOCX_ZERO to zero the last full trailing page of
large allocations that have been randomly assigned an offset of 0 when
--enable-cache-oblivious configure option is enabled.  This addresses a
special case missed in d260f442ce693de4351229027b37b3293fcbfd7d (Fix
xallocx(..., MALLOCX_ZERO) bugs.).
3 files changed