[truetype] Restore behavior of ft_var_load_hvvar

* src/truetype/ttgcvar.c (ft_var_load_hvvar): restore previous behavior

In a previous change [0] the behavior of `ft_var_load_hvvar` was changed
to not load the item variation store if it was at offset 0, but not
return an error when this happened. This broke any users, like
`tt_hvadvance_adjust`, that rely on successful completion of
`ft_var_load_hvvar` to imply that returned table's `itemStore` had been
initialized. This lead such users to dereference NULL.

This change appears to have been unintentional and unrelated to the
actual avar2 changes. As a result, fix these NULL dereferences by
restoring the code to always attempt to initialize the `itemStore`.

[0] ae4eb996 "[truetype] Add support for `avar` table 2.0 format."

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=53061

src/truetype/ttgxvar.c

@@ -929,15 +929,12 @@
       table = blend->hvar_table;
     }
 
-    if ( store_offset )
-    {
-      error = tt_var_load_item_variation_store(
-                face,
-                table_offset + store_offset,
-                &table->itemStore );
-      if ( error )
-        goto Exit;
-    }
+    error = tt_var_load_item_variation_store(
+              face,
+              table_offset + store_offset,
+              &table->itemStore );
+    if ( error )
+      goto Exit;
 
     if ( widthMap_offset )
     {