QUIC: allowed ngx_quic_frame_sendto() to return NGX_AGAIN. Previously, NGX_AGAIN returned by ngx_quic_send() was treated by ngx_quic_frame_sendto() as error, which triggered errors in its callers. However, a blocked socket is not an error. Now NGX_AGAIN is passed as is to the ngx_quic_frame_sendto() callers, which can safely ignore it.

--- a/src/event/quic/ngx_event_quic_output.c
+++ b/src/event/quic/ngx_event_quic_output.c
@@ -1233,7 +1233,7 @@ ngx_quic_frame_sendto(ngx_connection_t *
 
     sent = ngx_quic_send(c, res.data, res.len, path->sockaddr, path->socklen);
     if (sent < 0) {
-        return NGX_ERROR;
+        return sent;
     }
 
     path->sent += sent;