[lsan] Try to fix test failure due to compiler optimization

Summary:
The SanitizerCommon-lsan-x86_64-Linux test failed due to the address of
the very first allocation ending up in the stack through "delete[]".
Workaround this by performing another allocation. The issue was only
present with optimization enabled, the test would pass with -O0.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D46650

llvm-svn: 332020