Don't complain if $depth not set


git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@3727 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/unix/vncserver b/unix/vncserver
index a9b1b19..8f36d79 100755
--- a/unix/vncserver
+++ b/unix/vncserver
@@ -306,7 +306,7 @@
 	die "$prog: geometry $geometry is invalid\n";
     }
 
-    if (($depth < 8) || ($depth > 32)) {
+    if ($depth && (($depth < 8) || ($depth > 32))) {
 	die "Depth must be between 8 and 32\n";
     }
 }