MbedTLS: Treat 0 bytes read/written to socket properly

MbedTLS: Treat 0 bytes read/written to socket properly

At the moment, if a link quits in just the right (wrong [1]) way,
the quit reason will resemble:

    <-- foo (~bar@baz) has quit (Read error: (-0x0) )

This should resolve that.

[1] Peers should send a close_notify alert before abruptly shutting
    down their socket. This will result in a sane quit message:

    <-- foo (~bar@baz) has quit (Read error: (-0x7880) SSL -
    The peer notified us that the connection is going to be closed)

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