[ot-font] Fix leak

Fixes https://github.com/harfbuzz/harfbuzz/issues/1171

I'm glad we have leak-detector bots now.

src/hb-ot-post-table.hh

@@ -131,6 +131,7 @@ struct post
     {
       index_to_offset.fini ();
       free (gids_sorted_by_name.get ());
+      hb_blob_destroy (blob);
     }
 
     inline bool get_glyph_name (hb_codepoint_t glyph,