extract_if_dead: use a known working transfer when checking connections

Make sure that this function sets a proper "live" transfer for the
connection before calling the protocol-specific connection check
function, and then clear it again afterward as a non-used connection has
no current transfer.

Reported-by: Jeroen Ooms
Reviewed-by: Marcel Raad
Reviewed-by: Daniel Gustafsson
Fixes #3463
Closes #3464