Removed incorrect CHECK in SSL socket `send()`.
Removed incorrect CHECK in SSL socket `send()`.

The lambda placed on the event loop by the libevent SSL
socket's `send()` method previously used a `CHECK` to
ensure that the socket's `send_request` member was not
`nullptr`. This patch removes this check, since
`send_request` may become `nullptr` any time the socket
receives an EOF or ERROR event.

Note that the current handling of events is incorrect
also, but we do not attempt a fix here. To be specific,
reading EOF should not deal with send requests at all
(see MESOS-5999). Also, the ERROR events are not
differentiated between reading and writing. Lastly,
when we receive an EOF we do not ensure that the caller
can read the bytes that remain in the buffer!

Review: https://reviews.apache.org/r/50741/

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