test613: stop showing an error on missing output file

This test would show an error message if the output was missing during
the log post-processing step, but the message was not captured by the
test harness and wasn't useful since the normal golden log file
comparison would the problem more clearly.

tests/libtest/test613.pl

@@ -81,7 +81,7 @@ elsif ($ARGV[0] eq "postprocess")
 
     rmdir $dirname || die "$!";
 
-    if ($logfile) {
+    if ($logfile && -s $logfile) {
         # Process the directory file to remove all information that
         # could be inconsistent from one test run to the next (e.g.
         # file date) or may be unsupported on some platforms (e.g.