Avoid memory leak in ASan test

Summary:
Add missing free(3) for the malloc(3) call.

Detected on NetBSD with LSan.

Reviewers: joerg, mgorny, vitalybuka, dvyukov

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D67330

llvm-svn: 372460