_libssh2_channel_write: client spins on write when window full
_libssh2_channel_write: client spins on write when window full

When there's no window to "write to", there's no point in waiting for
the socket to become writable since it most likely just will continue to
be.

Patch-by: ncm
Fixes #258

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2