vnc: fix no-lock-key-sync strncmp() length

The no-lock-key-sync option is being parsed incorrectly because of an
outdated strcmp() length value.  Use the correct length so that invalid
option names do not match.

Reported-by: Dr David Alan Gilbert <davidagilbert@uk.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

ui/vnc.c

@@ -2763,7 +2763,7 @@ int vnc_display_open(DisplayState *ds, const char *display)
             password = 1; /* Require password auth */
         } else if (strncmp(options, "reverse", 7) == 0) {
             reverse = 1;
-        } else if (strncmp(options, "no-lock-key-sync", 9) == 0) {
+        } else if (strncmp(options, "no-lock-key-sync", 16) == 0) {
             lock_key_sync = 0;
 #ifdef CONFIG_VNC_SASL
         } else if (strncmp(options, "sasl", 4) == 0) {