Fix memory leak caused by fuzzing.

%DebugPromiseHandlePrologue expects to be followed up by %..Epilogue,
otherwise we get a leak.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/301553002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21489 ce2b1a6d-e550-0410-aec6-3dcde31c8c00