bss_dgram.c: Use BIO_ADDR_sockaddr() and BIO_ADDR_sockaddr_size()

bss_dgram.c: Use BIO_ADDR_sockaddr() and BIO_ADDR_sockaddr_size()

Especially, it's important to use BIO_ADDR_sockaddr_size() instead of
taking sizeof(addr), as BIO_ADDR is a union of several sockaddr variants
with different sizes, and some sendto() implementations are very picky
that the size is correct for the indicated sockaddr family.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20449)