Fix a gcc 11.2.0 warning

Fix a gcc 11.2.0 warning

gcc 11.2.0 is the default on Ubuntu 21.10. It emits a (spurious) warning
when compiling test/packettest.c, which causes --strict-warnings builds
to fail. A simple fix avoids the warning.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/16887)