BUG/MINOR: ssl-hello: make use of the null-terminated servername

In ssl_sock_switchctx_cbk(), the servername is copied into the trash
and null-terminated, but later in the call to strncpy() it's still used
as-is, so anything that follows it will be copied as well, which is not
really expected. Let's make the servername point to the trash after
sanitizing it, like ssl_sock_switchcbk_wolfSSL_cbk() does.

This can be backported to 2.6 since it was introduced with commit
a996763619 ("BUG/MINOR: ssl: Store client SNI in SSL context in case
of ClientHello error").

src/ssl_clienthello.c

@@ -448,6 +448,7 @@ sni_lookup:
 	for (i = 0; i < trash.size && i < servername_len; i++)
 		trash.area[i] = tolower((unsigned char)servername[i]);
 	trash.area[i] = 0;
+	servername = trash.area;
 
 	HA_RWLOCK_RDLOCK(SNI_LOCK, &s->sni_lock);
 	sni_ctx = ssl_sock_choose_sni_ctx(s, conn, trash.area, has_rsa_sig, has_ecdsa_sig);