[sanitizer] Fix a bug in AdjustStackSize().

If the user requests OS default stack size, do not adjust it to our minimum
stack size (which is usually much less than the OS default).

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204173 91177308-0d34-0410-b5e6-96231b3b80d8