Don't return without value here.

In particular, return 0 as this is a failure.
diff --git a/unix/xserver/hw/vnc/xvnc.c b/unix/xserver/hw/vnc/xvnc.c
index c668bde..627e797 100644
--- a/unix/xserver/hw/vnc/xvnc.c
+++ b/unix/xserver/hw/vnc/xvnc.c
@@ -408,7 +408,7 @@
 	    {
 		ErrorF("Invalid pixmap depth %d\n", depth);
 		UseMsg();
-		return;
+		return 0;
 	    }
 	    vfbPixmapDepths[depth] = TRUE;
 	    ret++;