BUG/MINOR: ssl: free correctly the sni in the backend SSL cache

__ssl_sock_load_new_ckch_instance() does not free correctly the SNI in
the session cache, it only frees the one in the current tid.

This bug was introduced with e18d4e8 ("BUG/MEDIUM: ssl: backend TLS
resumption with sni and TLSv1.3").

This fix must be backported where the mentionned commit was backported.
(all maintained versions).
diff --git a/src/ssl_ckch.c b/src/ssl_ckch.c
index 8216950..192ad6c 100644
--- a/src/ssl_ckch.c
+++ b/src/ssl_ckch.c
@@ -1799,7 +1799,7 @@
 
 		/* flush the session cache of the server */
 		for (i = 0; i < global.nbthread; i++) {
-			ha_free(&ckchi->server->ssl_ctx.reused_sess[tid].sni);
+			ha_free(&ckchi->server->ssl_ctx.reused_sess[i].sni);
 			ha_free(&ckchi->server->ssl_ctx.reused_sess[i].ptr);
 		}
 		HA_RWLOCK_WRUNLOCK(SSL_SERVER_LOCK, &ckchi->server->ssl_ctx.lock);