ovsdb-idl: Check if row->written is valid.

ovsdb-idl: Check if row->written is valid.

Commit cde3f1 "ovsdb-idl: Drop unnecessary allocation from
ovsdb_idl_txn_insert()." does lazy allocation of row->written
on the assumption that ovsdb_idl_txn_write() will handle it.
However, this isn't the case for empty rows created by something
like "ovs-vsctl init" so add a check before reading the bitfield.