Fix no-psk

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5667)
diff --git a/ssl/statem/extensions_srvr.c b/ssl/statem/extensions_srvr.c
index ee4cad1..90142eb 100644
--- a/ssl/statem/extensions_srvr.c
+++ b/ssl/statem/extensions_srvr.c
@@ -1047,6 +1047,7 @@
             return 0;
         }
 
+#ifndef OPENSSL_NO_PSK
         if(sess == NULL
                 && s->psk_server_callback != NULL
                 && idlen <= PSK_MAX_IDENTITY_LEN) {
@@ -1097,6 +1098,7 @@
                 OPENSSL_cleanse(pskdata, pskdatalen);
             }
         }
+#endif /* OPENSSL_NO_PSK */
 
         if (sess != NULL) {
             /* We found a PSK */