dgram: add `nread` assertion to `UDPWrap::OnRecv`

This asserts that the number of bytes received by the socket is less
than or equal to the size allocated for the temporary storage.

Signed-off-by: Darshan Sen <darshan.sen@postman.com>

PR-URL: https://github.com/nodejs/node/pull/40295


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>