Avoid calling menu_get_item when not needed in og_vocab_is_group_admin_context/og_vocab_query_taxonomy_vocabulary_load_...

og_vocab_query_taxonomy_vocabulary_load_multiple_alter can be called during installation/updates/cache rebuilds, which calls og_vocab_is_group_admin_context which calls menu_get_item which can trigger rebuild of the menu, which is quite expansive.

Is causing caches to be set too early while installation is happening (cause things like views, panel pages, etc. were being cached when not all modules enabled yet).