Small fixes for detect_invalid_pointer_pairs.

Summary:
One test-case uses a wrong operation (should be subtraction).
Second test-case should declare a global variables before a tested one
in order to guarantee we will find a red-zone.

Reviewers: kcc, jakubjelinek, alekseyshl

Reviewed By: alekseyshl

Subscribers: kubamracek

Differential Revision: https://reviews.llvm.org/D41481

llvm-svn: 323162