Don't assume an empty stderr.

GuardMalloc can print info to stderr, causing these tests to fail.
Since FileCheck errors on empty inputs, just add a bit of dummy
data to make it happy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203595 91177308-0d34-0410-b5e6-96231b3b80d8