repo2html will no longer return a non-zero exit status when cref-bad is empty

This bug was introduced by the last commit.

repo2html

@@ -107,6 +107,6 @@ grep -f"$cref_errlog_first" "$listcache" \
   2> >( sed 's/^/[Reprocessing] /g' >&2 )
 
 # if any invalid crefs remain, then they're bad
-[ -s "$cref_errlog" ] && {
+[ ! -s "$cref_errlog" ] || {
   echo "warning: bad cref(s); see $cref_errlog" >&2
 }