perf kwork: Use zfree() to avoid possibly accessing dangling pointers

When freeing a->b it is good practice to set a->b to NULL using
zfree(&a->b) so that when we have a bug where a reference to a freed 'a'
pointer is kept somewhere, we can more quickly cause a segfault if some
code tries to use a->b.

Convert one such case in the 'perf kwork' codebase.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Yang Jihong <yangjihong1@huawei.com>
Link: https://lore.kernel.org/lkml/Zjmc5EiN6zmWZj4r@x1Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-kwork.c

@@ -2230,7 +2230,7 @@ static int perf_kwork__top(struct perf_kwork *kwork)
 	perf_kwork__top_report(kwork);
 
 out:
-	free(kwork->top_stat.cpus_runtime);
+	zfree(&kwork->top_stat.cpus_runtime);
 	return ret;
 }