BUG/MEDIUM: lua: fix a segfault in txn:done() if called twice

BUG/MEDIUM: lua: fix a segfault in txn:done() if called twice

When called from an http ruleset, txn:done() can still crash the process
because it closes the stream without consuming pending data resulting in
the transaction's buffer representation to differ from the real buffer.

This patch also adjusts the transaction's state to indicate that it's
closed to be consistent with what's already done in redirect rules.