OPTIM: check: do not delay MUX for ALPN if SSL not active

OPTIM: check: do not delay MUX for ALPN if SSL not active

To ensure ALPN is properly applied on checks, MUX initialization is
delayed so that it is created on SSL handshake completion. However, this
does not check if SSL is really active for the connection.

This patch adjusts the condition so that MUX init is not delayed if SSL
is not active for the check connection. A similar process is already
conducted for normal connections via connect_server().

This must be backported up to 2.4. Despite not being a bug, it must be
backported for the following patch which fixes check ALPN inheritance
from server settings.