Fix crash caused by bad management of opener chains.
Fix crash caused by bad management of opener chains.

1. We need to reset (potentially used) chains after each mark analysis
   phase. This ensures that md_rollback() does not try to play with
   chains used in previous phases.

2. md_rollback() must never play with PTR_CHAIN.

mity/md4c,mity/md4c,mity/md4c