[safestack] Don't crash if stack size is not aligned as expected

Summary:
From runtime side looks it's OK to RoundUpTo to needed alignment as buffer is
going to be RoundUpTo to page size anyway.

Reviewers: eugenis, pcc

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D57866

llvm-svn: 353475