[scudo] Fix buildbot test error on ARM

Summary:
The assumption __sanitizer_get_heap_size() == 0 (introduced in D29341) at the
start of a program appears to be incorrect on some ARM machines
(SizeClassAllocator32).

This should fix the test while I investigate the issue.

Reviewers: kcc, alekseyshl

Reviewed By: alekseyshl

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D29516

llvm-svn: 294056