[sanitizer] When setting up shadow memory on iOS, fix handling the return...

[sanitizer] When setting up shadow memory on iOS, fix handling the return value of task_info on older OS versions

task_vm_info is a "revisioned" structure, new OS versions add fields to the end, and compatibility is based on the reported size. On older OS versions, min_address/max_address is not filled back. Let's handle that case. Unfortunately, we can't really write a test (as the failure only happens when on a specific OS version).

Differential Revision: https://reviews.llvm.org/D50275

llvm-svn: 340058