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 <matt@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19152)