HTTP/3: fixed segfault when using SSL certificates with variables.

HTTP/3: fixed segfault when using SSL certificates with variables.

A QUIC connection doesn't have c->log->data and friends initialized to sensible
values.  Yet, a request can be created in the certificate callback with such an
assumption, which leads to a segmentation fault due to null pointer dereference
in ngx_http_free_request().  The fix is to adjust initializing the QUIC part of
a connection such that it has all of that in place.

Further, this appends logging error context for unsuccessful QUIC handshakes:
- cannot load certificate .. while handling frames
- SSL_do_handshake() failed .. while sending frames