multi: call protocol handler done() if PROTOCONNECT or later

multi: call protocol handler done() if PROTOCONNECT or later

The protocol handlers' done() function would previous get called
unconditionally in multi_done(), no matter how far the easy handle's
state machine has transitioned.

This caused problems in IMAP which in imap_connect() initializes things
that the imap_done() function assumes has occured. I think that seems
like a correct assumption and we should rather make sure that the done()
function is only called if we have reached the PROTOCONNECT state.

This problem was found using OSS-Fuzz.

Assisted-by: Catena cyber
Closes #16681