Use 0 as inert object reference value instead of -1

Towards using Null object for nil objects.

src/hb-object-private.hh

@@ -41,7 +41,7 @@
 
 /* reference_count */
 
-#define HB_REFERENCE_COUNT_INERT_VALUE -1
+#define HB_REFERENCE_COUNT_INERT_VALUE 0
 #define HB_REFERENCE_COUNT_POISON_VALUE -0x0000DEAD
 #define HB_REFERENCE_COUNT_INIT {HB_ATOMIC_INT_INIT(HB_REFERENCE_COUNT_INERT_VALUE)}