* [libesl] Validate `Content-Length` in `esl_recv_event`.
`atol()` accepted negative values, allowing a remote ESL peer to cause
a one-byte heap underwrite (`Content-Length: -1`) or NULL-pointer
dereference (`Content-Length: -2`, since `esl_assert` compiles out
under `NDEBUG`). Reject negative and oversized values, and check
`malloc` failure instead of relying on `assert`.
Cap at `ESL_MAX_CONTENT_LENGTH` (16 MiB).
* [libesl] Add test_recv_event.
Dmitry Verenitsin