_libssh2_channel_read: store last error
_libssh2_channel_read: store last error

When the transport layer returns EAGAIN this function didn't call
_libssh2_error() which made the last_error not get set.

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