QUIC: always populate ngx_quic_cbs_recv_rcd() output arguments

Although uninitialized values aren't used in practice due to the
nature of the OpenSSL code flow, this violates the API contract.

Reported by lukefr09 on GitHub.

src/event/quic/ngx_event_quic_ssl.c

@@ -158,6 +158,7 @@ ngx_quic_cbs_recv_rcd(ngx_ssl_conn_t *ssl_conn,
         if (b->sync) {
             /* hole */
 
+            *data = NULL;
             *bytes_read = 0;
 
             break;
@@ -169,6 +170,9 @@ ngx_quic_cbs_recv_rcd(ngx_ssl_conn_t *ssl_conn,
         break;
     }
 
+    *data = NULL;
+    *bytes_read = 0;
+
     return 1;
 }