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 d260f442ce (Fix
xallocx(..., MALLOCX_ZERO) bugs.).

include/jemalloc/internal/jemalloc_internal.h.in

@@ -317,6 +317,10 @@ typedef unsigned szind_t;
 #define	PAGE		((size_t)(1U << LG_PAGE))
 #define	PAGE_MASK	((size_t)(PAGE - 1))
 
+/* Return the page base address for the page containing address a. */
+#define	PAGE_ADDR2BASE(a)						\
+	((void *)((uintptr_t)(a) & ~PAGE_MASK))
+
 /* Return the smallest pagesize multiple that is >= s. */
 #define	PAGE_CEILING(s)							\
 	(((s) + PAGE_MASK) & ~PAGE_MASK)