Fix compute_size_with_overflow().
Fix compute_size_with_overflow().

Fix compute_size_with_overflow() to use a high_bits mask that has the
high bits set, rather than the low bits.  This regression was introduced
by 5154ff32ee8c37bacb6afd8a07b923eb33228357 (Unify the allocation
paths).

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc