Upstream: clearing of u->peer.connection on close.
Upstream: clearing of u->peer.connection on close.

This fixes crashes observed with some 3rd party balancer modules.  Standard
balancer modules (round-robin and ip hash) explicitly set pc->connection
(aka u->peer.connection) to NULL and aren't affected.

hy0kl/nginx,firebase/nginx,hy0kl/nginx,firebase/nginx,hy0kl/nginx,firebase/nginx,firebase/nginx