BUG/MEDIUM: connections: Don't forget to set xprt_ctx to NULL on close.

In conn_xprt_close(), after calling xprt->close(), don't forget to set
conn->xprt_ctx to NULL, or we may attempt to reuse the now-free'd
conn->xprt_ctx if the connection failed and we're retrying it.

include/proto/connection.h

@@ -99,6 +99,7 @@ static inline void conn_xprt_close(struct connection *conn)
 	if ((conn->flags & (CO_FL_XPRT_READY|CO_FL_XPRT_TRACKED)) == CO_FL_XPRT_READY) {
 		if (conn->xprt->close)
 			conn->xprt->close(conn, conn->xprt_ctx);
+		conn->xprt_ctx = NULL;
 		conn->flags &= ~CO_FL_XPRT_READY;
 	}
 }