Don't junk-fill reallocations unless usize changes.

Don't junk fill reallocations for which the request size is less than
the current usable size, but not enough smaller to cause a size class
change.  Unlike malloc()/calloc()/realloc(), *allocx() contractually
treats the full usize as the allocation, so a caller can ask for zeroed
memory via mallocx() and a series of rallocx() calls that all specify
MALLOCX_ZERO, and be assured that all newly allocated bytes will be
zeroed and made available to the application without danger of allocator
mutation until the size class decreases enough to cause usize reduction.