_libssh2_channel_write() first reads outstanding packets before writing
new data. If it reads a key exchange request, it will immediately start
key re-exchange, which will require sending a response. If the output
socket is full, this will result in a return from
_libssh2_transport_read() of LIBSSH2_ERROR_EAGAIN. In order not to block
a write because there is no data to read, this error is explicitly
ignored and the code continues marshalling a packet for sending. When it
is sent, the remote end immediately drops the connection because it was
expecting a continuation of the key exchange, but got a data packet.
This change adds the same check for key exchange to
_libssh2_transport_send() that is in _libssh2_transport_read(). This
ensures that key exchange is completed before any data packet is sent.
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