perf tools: Introduce zfree

For the frequent idiom of:

   free(ptr);
   ptr = NULL;

Make it expect a pointer to the pointer being freed, so that it becomes
clear at first sight that the variable being freed is being modified.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-pfw02ezuab37kha18wlut7ir@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index dab98b5..106a5e5 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -185,8 +185,7 @@
 
 static void perf_evsel__free_stat_priv(struct perf_evsel *evsel)
 {
-	free(evsel->priv);
-	evsel->priv = NULL;
+	zfree(&evsel->priv);
 }
 
 static int perf_evsel__alloc_prev_raw_counts(struct perf_evsel *evsel)
@@ -208,8 +207,7 @@
 
 static void perf_evsel__free_prev_raw_counts(struct perf_evsel *evsel)
 {
-	free(evsel->prev_raw_counts);
-	evsel->prev_raw_counts = NULL;
+	zfree(&evsel->prev_raw_counts);
 }
 
 static void perf_evlist__free_stats(struct perf_evlist *evlist)