expr: Fix memory leak reported by valgrind.

expr: Fix memory leak reported by valgrind.

Testcase 1728: ovn -- 5-term mixed expression normalization.
    expr_clone_cmp (expr.c:1259)
    expr_clone (expr.c:1284)
    expr_clone_andor (expr.c:1271)
    expr_clone (expr.c:1288)
    expr_normalize_and (expr.c:2137)
    test_tree_shape_exhaustively (test-ovn.c:926)

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Co-authored-by: Daniele Di Proietto <diproiettod@vmware.com>
Signed-off-by: Russell Bryant <russell@ovn.org>