BUG/MINOR: backend: do not try to install a mux when the connection failed

BUG/MINOR: backend: do not try to install a mux when the connection failed

If si_connect() failed, do not try to install the mux nor to complete
the operations or add the connection to an idle list, and abort quickly
instead. No obvious side effects were identified, but continuing to
allocate some resources after something has already failed seems risky.

This was a result of a prior fix which already wanted to push this code
further : aa089d80b ("BUG/MEDIUM: server: Defer the mux init until after
xprt has been initialized.") but it ought to have pushed it even further
to maintain the error check just after si_connect().

To be backported to 2.0 and 1.9.