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 <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>