releasing_test failed

The commit I use is `3dda5d0`

----

I use gcc 7.3.1 to build and run the test:
`CXX=/usr/bin/g++ CC=/usr/bin/gcc bazel --output_user_root=/data/bazel-cache test //tcmalloc/...`

And there's one failure. Here's the log:
```
cat /data/bazel-cache/53be17487e92ab49f9a9a0a4d546d9a6/execroot/com_google_tcmalloc/bazel-out/k8-fastbuild/testlogs/tcmalloc/testing/releasing_test/test.log
exec ${PAGER:-/usr/bin/less} "$0" || exit 1
Executing tests from //tcmalloc/testing:releasing_test
-----------------------------------------------------------------------------
tcmalloc/testing/releasing_test.cc:142] Unmapped Memory [Before] 155189248
tcmalloc/testing/releasing_test.cc:144] Unmapped Memory [After ] 2535456768
tcmalloc/testing/releasing_test.cc:146] Unmapped Memory [Diff ] 2380267520
tcmalloc/testing/releasing_test.cc:148] Memory Usage [Before] 2358042624
tcmalloc/testing/releasing_test.cc:150] Memory Usage [After ] 51367936
tcmalloc/testing/releasing_test.cc:152] Memory Usage [Diff ] 2306674688
(after_unmapped - before_unmapped) != (before - after):18446744071794851840] 2306674688 @ 0x40e8da 0x7f94f4340c05
```