MINOR: mux-h1: use explicit __objt_server on idle conn reinsert

MINOR: mux-h1: use explicit __objt_server on idle conn reinsert

When dealing with a backend connection, H1 mux IO handler must reinsert
it in its idle list pool if it was extracted from it at the beginning.
This is the case if conn_in_list is true.

On reinsert, idle list pool is retrieved via the server instance
accessible from <conn.target>. Replace objt_server usage with
__objt_server as an idle connection is always attached to a server. This
ensures that there is no issue when using _srv_add_idle() then.

This should fix coverity report from github issue #2810.