ui: fix handling of NULL SASL server data

The code is supposed to distinguish between SASL server data that
is NULL, vs non-NULL but zero-length. It was incorrectly checking
the 'serveroutlen' variable, rather than 'serverout' though, so
failing to distinguish the cases.

Fortunately we can fix this without breaking compatibility with
clients, as clients already know how to decode the input data
correctly.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

ui/vnc-auth-sasl.c

@@ -289,9 +289,10 @@ static int protocol_client_auth_sasl_step(VncState *vs, uint8_t *data, size_t le
         goto authabort;
     }
 
-    if (serveroutlen) {
+    if (serverout) {
         vnc_write_u32(vs, serveroutlen + 1);
-        vnc_write(vs, serverout, serveroutlen + 1);
+        vnc_write(vs, serverout, serveroutlen);
+        vnc_write_u8(vs, '\0');
     } else {
         vnc_write_u32(vs, 0);
     }
@@ -410,9 +411,10 @@ static int protocol_client_auth_sasl_start(VncState *vs, uint8_t *data, size_t l
         goto authabort;
     }
 
-    if (serveroutlen) {
+    if (serverout) {
         vnc_write_u32(vs, serveroutlen + 1);
-        vnc_write(vs, serverout, serveroutlen + 1);
+        vnc_write(vs, serverout, serveroutlen);
+        vnc_write_u8(vs, '\0');
     } else {
         vnc_write_u32(vs, 0);
     }