BUG/MEDIUM: stconn: don't set the type before allocation succeeds

BUG/MEDIUM: stconn: don't set the type before allocation succeeds

There's an occasional crash that can be triggered in sc_detach_endp()
when calling conn->mux->detach() upon memory allocation error. The
problem in fact comes from sc_attach_mux(), which doesn't reset the
sc type flags upon tasklet allocation failure, leading to an attempt
at detaching an incompletely initialized stconn. Let's just attach
the sc after the tasklet allocation succeeds, not before.

This must be backported to 2.6.