When libssh2_transport_write() is called to continue sending a
partially sent packet the write direction flag must not be cleared
until the previous packet has been completely sent, or the app would
hang if the packet still isn't sent completely, since select() gets
called by the internal blocking emulation layer in libssh2 but would
then not be watching the socket for writability.
Clear the flag only once processing of previous packet data is
complete and a new packet is about to be prepared.
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