fix nil pointer issue

proxy_local/http2_proxy.go

@@ -239,10 +239,12 @@ func (p *clientConn) MarkDead(conn *http2.ClientConn) {
 	p.lock.Lock()
 	defer p.lock.Unlock()
 
+	if p.conn != nil {
 		log.Errorln("mark dead")
 		p.conn.Close()
 		p.conn = nil
 	}
+}
 
 var debug bool
 var insecure bool