Fix seff to handle NULL variables.

contribs/seff/seff

@@ -219,5 +219,6 @@ sub kbytes2str {
     my $exp = int(log($kbytes) / log($mul));
     my @pre = qw/ M G T P E /;
     my $pre = $pre[$exp-1];
-    return sprintf("%.2f %sB", ($kbytes / pow($mul, $exp)), $pre);
+
+    return sprintf("%.2f %sB", ($kbytes / pow($mul, $exp)), $pre ? $pre : "");
 }