netlink_delinearize: memleak when listing ct event rule

netlink_delinearize: memleak when listing ct event rule

listing a ruleset containing:

	ct event set new,related,destroy,label

results in memleak:

 Direct leak of 3672 byte(s) in 27 object(s) allocated from:
    #0 0x7fa5465c0330 in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xe9330)
    #1 0x7fa54233772c in xmalloc /home/.../devel/nftables/src/utils.c:36
    #2 0x7fa5423378eb in xzalloc /home/.../devel/nftables/src/utils.c:75
    #3 0x7fa5422488c6 in expr_alloc /home/.../devel/nftables/src/expression.c:45
    #4 0x7fa54224fb91 in binop_expr_alloc /home/.../devel/nftables/src/expression.c:698
    #5 0x7fa54224ddf8 in bitmask_expr_to_binops /home/.../devel/nftables/src/expression.c:512
    #6 0x7fa5423102ca in expr_postprocess /home/.../devel/nftables/src/netlink_delinearize.c:2448

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>