Proxy: reset pending control frames on HTTP/2 upstream reinit.

Previously, ctx->out was not cleared in ngx_http_proxy_v2_reinit_request(),
which could cause stale HTTP/2 control frames (SETTINGS ACK, PING ACK,
WINDOW_UPDATE) queued for the old upstream connection to be sent to a new
upstream connection during a retry.

src/http/modules/ngx_http_proxy_v2_module.c

@@ -946,6 +946,7 @@ ngx_http_proxy_v2_reinit_request(ngx_http_request_t *r)
     ctx->connection = NULL;
     ctx->in = NULL;
     ctx->busy = NULL;
+    ctx->out = NULL;
 
     return NGX_OK;
 }