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>

tools/perf/util/thread_map.c

@@ -9,6 +9,7 @@
 #include "strlist.h"
 #include <string.h>
 #include "thread_map.h"
+#include "util.h"
 
 /* Skip "." and ".." directories */
 static int filter(const struct dirent *dir)
@@ -138,8 +139,7 @@ out_free_namelist:
 	free(namelist);
 
 out_free_closedir:
-	free(threads);
-	threads = NULL;
+	zfree(&threads);
 	goto out_closedir;
 }
 
@@ -210,8 +210,7 @@ out_free_namelist:
 	free(namelist);
 
 out_free_threads:
-	free(threads);
-	threads = NULL;
+	zfree(&threads);
 	goto out;
 }
 
@@ -262,8 +261,7 @@ out:
 	return threads;
 
 out_free_threads:
-	free(threads);
-	threads = NULL;
+	zfree(&threads);
 	goto out;
 }