bio_dgram_test.c: Fix warning from older clang compilers
bio_dgram_test.c: Fix warning from older clang compilers

Older clang compilers warn about the initializer:

test/bio_dgram_test.c:107:29: error: suggest braces around initialization
 of subobject [-Werror,-Wmissing-braces]
    struct in6_addr ina6 = {0};
                            ^
                            {}

Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
Reviewed-by: Hugo Landau <ce9fa1e55a72fc9b85dd06e47ea2c07848fb7d2c@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19152)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl