Correctly mark NullPool const

Saves some more code size as well!

src/hb-open-type-private.hh

@@ -133,7 +133,7 @@ static inline Type& StructAfter(TObject &X)
 
 #define HB_NULL_POOL_SIZE 264
 static_assert (HB_NULL_POOL_SIZE % sizeof (void *) == 0, "Align HB_NULL_POOL_SIZE.");
-extern HB_INTERNAL const void *_hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
+extern HB_INTERNAL const void * const _hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
 
 /* Generic nul-content Null objects. */
 template <typename Type>

src/hb-ot-layout.cc

@@ -39,7 +39,7 @@
 #include "hb-ot-map-private.hh"
 
 
-const void *OT::_hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
+const void * const OT::_hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)] = {};
 
 
 hb_ot_layout_t *

src/main.cc

@@ -38,7 +38,7 @@
 
 using namespace OT;
 
-const void *OT::_hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
+const void * const OT::_hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)] = {};
 
 int
 main (int argc, char **argv)