update allocated message buffer size to avoid hardcoded number.

[bovine]

The size of zmq_msg_t was increased from 32 bytes to 64 bytes in v4.2.0-rc1 about 2.5 years ago:
zeromq/libzmq@9019403

Failure to allocate a large enough buffer causes memory corruption and typically ckfree() faults when trying to close the message.

[bovine]

It was actually increased from 32 bytes to 40, then 48, then 64 over several different commits over the years.

It was 32 bytes here: zeromq/libzmq@7bf516d