Index: vnc.c
===================================================================
--- vnc.c	(revision 4615)
+++ vnc.c	(working copy)
@@ -854,6 +854,11 @@
 {
     int i;
     vs->need_update = 1;
+
+    if (x_position + w > vs->ds->width)  w = vs->ds->width  - x_position;
+    if (y_position + h > vs->ds->height) h = vs->ds->height - y_position;
+    if (w <= 0 || h <= 0) return;
+    
     if (!incremental) {
 	char *old_row = vs->old_data + y_position * vs->ds->linesize;