QUIC: better sockaddr initialization.

QUIC: better sockaddr initialization.

The qsock->sockaddr field is a ngx_sockaddr_t union, and therefore can hold
any sockaddr (and union members, such qsock->sockaddr.sockaddr, can be used
to access appropriate variant of the sockaddr).  It is better to set it via
qsock->sockaddr itself though, and not qsock->sockaddr.sockaddr, so static
analyzers won't complain about out-of-bounds access.

Prodded by Coverity (CID 1530403).