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/ui/browser.c b/tools/perf/ui/browser.c
index 94223d40..d11541d 100644
--- a/tools/perf/ui/browser.c
+++ b/tools/perf/ui/browser.c
@@ -256,8 +256,7 @@
 	__ui_browser__show_title(browser, title);
 
 	browser->title = title;
-	free(browser->helpline);
-	browser->helpline = NULL;
+	zfree(&browser->helpline);
 
 	va_start(ap, helpline);
 	err = vasprintf(&browser->helpline, helpline, ap);
@@ -272,8 +271,7 @@
 {
 	pthread_mutex_lock(&ui__lock);
 	ui_helpline__pop();
-	free(browser->helpline);
-	browser->helpline = NULL;
+	zfree(&browser->helpline);
 	pthread_mutex_unlock(&ui__lock);
 }