Fix lg_chunk clamping to take into account cache-oblivious large
allocation. This regression only resulted in incorrect behavior if
!config_fill (false unless --disable-fill specified) and
config_cache_oblivious (true unless --disable-cache-oblivious
specified).
This regression was introduced by
8a03cf039cd06f9fa6972711195055d865673966 (Implement cache index
randomization for large allocations.), which was first released in
4.0.0.
This resolves#555.