Appease -Werror=maybe-uninitialized

Appease -Werror=maybe-uninitialized

test/bad_dtls_test.c: In function 'validate_client_hello':
test/bad_dtls_test.c:128:33: error: 'u' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     if (!PACKET_get_1(&pkt, &u) || u != SSL3_RT_HANDSHAKE)
                                 ^
Apparently -O1 does not perform sufficient optimization to ascertain
that PACKET_get_1 will always initialize u if it returns true.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4518)