zmq_poller_add/zmq_poller_modify should reject invalid events arguments

[sigiesec]

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Issue description

zmq_poller_add/zmq_poller_modify should reject invalid events arguments, rather than accepting them silently.

Environment

• libzmq version (commit hash if unreleased): 4.2.5
• OS: any

Minimal test code / Steps to reproduce the issue

void *ctx = zmq_ctx_new();
void *socket = zmq_socket(ctx, ZMQ_DEALER);
void *poller = zmq_poller_new();
int res = zmq_poller_add(poller, socket, nullptr, 42);

What's the actual result? (include assertion message & call stack if applicable)

res == 0

What's the expected result?

res == -1 && zmq_errno() == EINVAL