BUG/MINOR: quic-be: missing connection stream closure upon TLS alert to send

BUG/MINOR: quic-be: missing connection stream closure upon TLS alert to send

This is the same issue as the one fixed by this commit:
   BUG/MINOR: quic-be: handshake errors without connection stream closure
But this time this is when the client has to send an alert to the server.
The fix consists in creating the mux after having set the handshake connection
error flag and error_code.

This bug was revealed by ssl/set_ssl_cafile.vtc reg test.

Depends on this commit:
     MINOR: quic: avoid code duplication in TLS alert callback

Must be backported to 3.3