[scudo] Fix the calculating of memory group usage

In SizeClassAllocator64, the boundary of a memory group may not align to
the region begin. Which means the begin addr of a memory group may
smaller than region begin. This leads to wrong judgement of memory group
usage.

Differential Revision: https://reviews.llvm.org/D136898

GitOrigin-RevId: 6130d67f70ad2e063b4407b6ee31c7db19464fee
2 files changed