QUIC: fixed padding calculation.

Sometimes, QUIC packets need to be of certain (or minimal) size.  This is
achieved by adding PADDING frames.  It is possible, that adding padding will
affect header size, thus forcing us to recalculate padding size once more.

src/event/quic/ngx_event_quic_output.c

@@ -368,6 +368,9 @@ ngx_quic_output_packet(ngx_connection_t *c, ngx_quic_send_ctx_t *ctx,
     }
 
     if (out.len < pad_len) {
+        /* compensate for potentially enlarged header in Length bytes */
+        pad_len -= ngx_quic_create_header(&pkt, NULL, pad_len, NULL)
+                   - ngx_quic_create_header(&pkt, NULL, out.len, NULL);
         ngx_memset(p, NGX_QUIC_FT_PADDING, pad_len - out.len);
         out.len = pad_len;
     }