Use nullptr instead of NULL
diff --git a/src/hb-mutex-private.hh b/src/hb-mutex-private.hh
index ed27035..49ed10e 100644
--- a/src/hb-mutex-private.hh
+++ b/src/hb-mutex-private.hh
@@ -68,7 +68,7 @@
#include <pthread.h>
typedef pthread_mutex_t hb_mutex_impl_t;
#define HB_MUTEX_IMPL_INIT PTHREAD_MUTEX_INITIALIZER
-#define hb_mutex_impl_init(M) pthread_mutex_init (M, NULL)
+#define hb_mutex_impl_init(M) pthread_mutex_init (M, nullptr)
#define hb_mutex_impl_lock(M) pthread_mutex_lock (M)
#define hb_mutex_impl_unlock(M) pthread_mutex_unlock (M)
#define hb_mutex_impl_finish(M) pthread_mutex_destroy (M)
