`read_packet()` passed a peer-supplied `Content-Length` straight to
`switch_zmalloc(body, clen + 1)`. Huge values drove `calloc` failure
and `switch_zmalloc` `abort()`-ed the daemon.
- Cap `Content-Length` at 16 MiB; reject negatives.
- Destroy the partially-built `*event` at the new rejection site and
at the existing body-recv failure path so callers don't leak it.
- Add `test_mod_event_socket` covering `INT_MAX`, above-cap,
negative, `atoi`-overflow, zero, and valid-non-zero-body cases.
