Add a constructor for hb_prealloced_array_t

Fixes build with MSVC.

src/hb-private.hh

@@ -240,6 +240,8 @@ struct hb_prealloced_array_t {
   Type *array;
   Type static_array[StaticSize];
 
+  hb_prealloced_array_t (void) { memset (this, 0, sizeof (*this)); }
+
   inline Type& operator [] (unsigned int i) { return array[i]; }
   inline const Type& operator [] (unsigned int i) const { return array[i]; }