Fix t/op/dump.t warning in skip case.

When the whole test is skipped, the END block that does a chdir
to $start throws a warning because $start is not defined.  So
check for definedness.

1 files changed, 1 insertions, 1 deletions

diff --git a/t/op/dump.t b/t/op/dump.t
index 270b58c3a8..a2e320f4da 100644
--- a/t/op/dump.t
+++ b/t/op/dump.t
@@ -52,5 +52,5 @@ print qq(B);
 PROG
 
 END {
-  chdir $start;
+  chdir $start if defined $start;
 }