perf affinity: Allow passing a NULL arg to affinity__cleanup()

Just like with free(), NULL is checked to avoid having all callers do
it.

Its convenient for when not using affinity setup/cleanup for dummy CPU
maps, i.e. CPU maps for pid targets.

Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220117160931.1191712-2-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>