Fix (harmless) memory leak in the test harness.

Change-Id: Ia0daaaaf464cfa0e9d563d7f376ce2bb2e338685
Reviewed-on: https://boringssl-review.googlesource.com/1560
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>