Add hb_addressof() and use it to fix bug after hb_bytes_t merge

We cannot take address hb_bytes_t direction.  We need to use the
newly added hb_addressof(), ala std::addressof().

src/hb-dsalgs.hh

@@ -234,6 +234,15 @@ hb_ctz (T v)
  * Tiny stuff.
  */
 
+template <typename T>
+T* hb_addressof (T& arg)
+{
+  /* https://en.cppreference.com/w/cpp/memory/addressof */
+  return reinterpret_cast<T*>(
+	   &const_cast<char&>(
+	      reinterpret_cast<const volatile char&>(arg)));
+}
+
 /* ASCII tag/character handling */
 static inline bool ISALPHA (unsigned char c)
 { return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'); }

src/hb-ot-post-table.hh

@@ -168,7 +168,8 @@ struct post
       }
 
       hb_bytes_t st (name, len);
-      const uint16_t *gid = (const uint16_t *) hb_bsearch_r (&st, gids, count, sizeof (gids[0]), cmp_key, (void *) this);
+      const uint16_t *gid = (const uint16_t *) hb_bsearch_r (hb_addressof (st), gids, count,
+							     sizeof (gids[0]), cmp_key, (void *) this);
       if (gid)
       {
 	*glyph = *gid;