BUG/MINOR: quic: Fix another buffer overflow with sockaddr_in46

BUG/MINOR: quic: Fix another buffer overflow with sockaddr_in46

Very similarly to what was fixed with commit
63f853957af3ee062493bb3700f964ce456125b0, we cast a sockaddr_in46 in
quic_dgram_parse() to sockaddr_storage while providing source and
destination addresses to qc_handle_conn_migration(), which will then
copy the whole sockaddr_storage, thus reading memory past what was
provided.
While this most likely won't have any impact, let's do the right thing,
and use in46un_to_addr() to generate a real sockaddr_storage.
This does not need to be backported.