Add check to make sure that NS (swapmod) file is a plain file and

accessible.

tbsetup/swapexp.in

@@ -2,7 +2,7 @@
 
 #
 # EMULAB-COPYRIGHT
-# Copyright (c) 2000-2003 University of Utah and the Flux Group.
+# Copyright (c) 2000-2004 University of Utah and the Flux Group.
 # All rights reserved.
 #
 
@@ -187,6 +187,10 @@ if ($inout eq "modify" && @ARGV > 2) {
     else {
 	die("Tainted nsfile name: $tempnsfile");
     }
+    if (! -f $tempnsfile || -z $tempnsfile) {
+	die("*** $0:\n".
+	    "    $tempnsfile does not look like an NS file!\n");
+    }
     $modnsfile = "$eid-modify.ns";
 }