Avoid calling closed? outside of synchronize block (#534)

* Avoid calling closed? outside of synchronize block

`closed?` calls `process` on the connection which is not safe because
we have not synchronised the connection pool. Another thread might
concurrently checkout the connection and start sending commands as well.

* Update lib/sshkit/backends/connection_pool/cache.rb

Co-authored-by: Matt Brictson <matt@123mail.org>

---------

Co-authored-by: Matt Brictson <matt@123mail.org>

lib/sshkit/backends/connection_pool/cache.rb

@@ -36,8 +36,8 @@ def push(conn)
   def evict
     # Peek at the first connection to see if it is still fresh. If so, we can
     # return right away without needing to use `synchronize`.
-    first_expires_at, first_conn = connections.first
-    return if (first_expires_at.nil? || fresh?(first_expires_at)) && !closed?(first_conn)
+    first_expires_at, _first_conn = connections.first
+    return if (first_expires_at.nil? || fresh?(first_expires_at))
 
     connections.synchronize do
       fresh, stale = connections.partition do |expires_at, conn|