(ITS#8157) txn can be useless after mdb_txn_renew()

[h.b.furuseth@usit.uio.no]

Full_Name: Hallvard B Furuseth
Version: LMDB 0.9.14
OS: 
URL: 
Submission from: (NULL) (81.191.45.5)
Submitted by: hallvard


mdb_txn_renew() may initialize txn halfway and then fail.
After that, it refuses to work even when it should have
succeeded, since the txn looks active (mt_dbxs != NULL).
Fixing.