perf kmem: Fix compiler warning about may be accessing uninitialized variable

perf kmem: Fix compiler warning about may be accessing uninitialized variable

The last argument to strtok_r doesn't need to be initialized, its just a
placeholder to make this routine reentrant, but gcc doesn't know about
that and complains, breaking the build, fix it by setting it to NULL.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-8e8rgbg3aom9uarsyqjrsctg@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>