ovsdb-idl: Fix NULL deref reported by Coverity.

When 'datum.values' or 'datum.keys' is NULL, some code path calling
into ovsdb_idl_txn_write__ triggers NULL deref.

An example:
ovsrec_open_vswitch_set_cur_cfg(const struct ovsrec_open_vswitch
{
    struct ovsdb_datum datum;
    union ovsdb_atom key;

    datum.n = 1;
    datum.keys = &key;

    key.integer = cur_cfg;
//  1. assign_zero: Assigning: datum.values = NULL.
    datum.values = NULL;
//  CID 1421356 (openvswitch#1 of 1): Explicit null dereferenced (FORWARD_NULL)
//  2. var_deref_model: Passing &datum to ovsdb_idl_txn_write_clone,\
//     which dereferences null datum.values.
    ovsdb_idl_txn_write_clone(&row->header_, &ovsrec_open_vswitch_col
}

And with the following calls:
ovsdb_idl_txn_write_clone
  ovsdb_idl_txn_write__
    6. deref_parm_in_call: Function ovsdb_datum_destroy dereferences
       datum->values
    ovsdb_datum_destroy

Reviewed-by: Yifeng Sun <[email protected]>
Signed-off-by: William Tu <[email protected]>

lib/ovsdb-idl.c

@@ -4449,7 +4449,8 @@ ovsdb_idl_txn_write__(const struct ovsdb_idl_row *row_,
      * transaction only does writes of existing values, without making any real
      * changes, we will drop the whole transaction later in
      * ovsdb_idl_txn_commit().) */
-    if (write_only && ovsdb_datum_equals(ovsdb_idl_read(row, column),
+    if (datum->keys && datum->values &&
+        write_only && ovsdb_datum_equals(ovsdb_idl_read(row, column),
                                          datum, &column->type)) {
         goto discard_datum;
     }