coverity 12559 Uninitialized pointer read In ldbm_back_modify(): Reads an uninitialized pointer or its target
    
    coverity 12559 Uninitialized pointer read In ldbm_back_modify(): Reads an uninitialized pointer or its target
    Have to call dblayer_txn_init() before testing or freeing the transaction
    Reviewed by: mareynol (Thanks!)