Fixed an obvious typo in libvncserver's clipping.

diff --git a/libvncserver b/libvncserver
index 30e11cfcd39a8bf7ec3038bff6803d031a4d4dfe..6d56232bdbd6997ef430ff4bc6fb3717307ec113 160000 (submodule)
--- a/libvncserver
+++ b/libvncserver
@@ -1 +1 @@
-Subproject commit 30e11cfcd39a8bf7ec3038bff6803d031a4d4dfe
+Subproject commit 6d56232bdbd6997ef430ff4bc6fb3717307ec113