drd/tests/thread_namedrd/tests/thread_name: Ensure mutex address uniqueness (see also #331847)

From: Ivo Raisr <ivosh@ivosh.net>


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13854 a5019735-40e9-0310-863c-91ae7b9d1cf9