box: fix memory leak in lbox_key_def_new and luaT_key_def_merge

There was a typo in the commit 55295f5f ("box: populate
index_object.parts with key_def module methods"):
the function luaT_push_key_def() was erroneously used instead
of luaT_push_key_def_nodup().

Follow-up #7356

NO_DOC=bug fix
NO_TEST=memory leak
NO_CHANGELOG=unreleased