ovsdb-idlc: Fix memory leak reported by Coverity.

ovsdb-idlc: Fix memory leak reported by Coverity.

An exmplae pattern shown below:
void
ovsrec_ct_zone_index_set_external_ids(const struct ovsrec_ct_zone...
{
//  1. alloc_fn: Storage is returned from allocation function xmalloc.
//  2. var_assign: Assigning: datum = storage returned from xmalloc(24UL).
    struct ovsdb_datum *datum = xmalloc(sizeof(struct ovsdb_datum));

//  3. Condition external_ids, taking false branch.
    if (external_ids) {
		...
    } else {
// 	4. noescape: Resource datum is not freed or pointed-to in ovsdb_datum_init_empty.
        ovsdb_datum_init_empty(datum);
    }
// 	5. noescape: Resource datum is not freed or pointed-to in ovsdb_idl_index_write.
    ovsdb_idl_index_write(CONST_CAST(struct ovsdb_idl_row *, &row->header_),
                          &ovsrec_ct_zone_columns[OVSREC_CT_ZONE_COL_EXTERNAL_IDS],
                          datum,
                          &ovsrec_table_classes[OVSREC_TABLE_CT_ZONE]);

// CID 1420856 (#1 of 1): Resource leak (RESOURCE_LEAK)
// 6. leaked_storage: Variable datum going out of scope leaks the storage it
      points to.
Fix it by freeing the datum.

Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>