url: skip to-be-closed connections when pipelining

No longer attempt to use "doomed" to-be-closed connections when
pipelining. Prior to this change connections marked for deletion (e.g.
timeout) would be erroneously used, resulting in sporadic crashes.

As originally reported and fixed by Carlo Wood (origin unknown).

Bug: https://github.com/curl/curl/issues/627
Reported-by: Rider Linden

Closes https://github.com/curl/curl/pull/1075


Participation-by:  <nopjmp@users.noreply.github.com>