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 <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Yang Jihong <[email protected]>
Link: https://lore.kernel.org/lkml/Zjmc5EiN6zmWZj4r@x1
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: acmel

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;
 }