ext/session_ticket: avoid calling memcpy on overlapping memory areas

ext/session_ticket: avoid calling memcpy on overlapping memory areas

In _gnutls_encrypt_session_ticket, ticket.encrypted_state is allocated
from ticket_data->data, thus those memory areas may overlap.  Using
memcpy here leads to undefined behavior.

Spotted by valgrind run on ppc64le.

==95231== Source and destination overlap in memcpy(0x47ce3a2, 0x47ce3a2, 160)
==95231==    at 0x408A840: memcpy (vg_replace_strmem.c:1023)
==95231==    by 0x424EE9F: pack_ticket (session_ticket.c:139)
==95231==    by 0x424FA4F: _gnutls_encrypt_session_ticket (session_ticket.c:335)
==95231==    by 0x4199E3B: generate_session_ticket (session_ticket.c:249)
==95231==    by 0x419A333: _gnutls13_send_session_ticket (session_ticket.c:307)
==95231==    by 0x40F8817: _gnutls13_handshake_server (handshake-tls13.c:511)
==95231==    by 0x4110DEB: handshake_server (handshake.c:3331)
==95231==    by 0x410C70B: gnutls_handshake (handshake.c:2727)
==95231==    by 0x10009EBF: retry_handshake (serv.c:1306)
==95231==    by 0x1000AB67: tcp_server (serv.c:1500)
==95231==    by 0x10009E5B: main (serv.c:1297)
==95231==

Signed-off-by: Daiki Ueno <dueno@redhat.com>