purple: fix FT behavior with NULL read/write callbacks

purple: fix FT behavior with NULL read/write callbacks

If we don't want to override how libpurple handles socket reading and
writing (i.e. no ft_read or ft_write callback set in sipe_file_transfer
structure), we must not call purple_xfer_set_read_fnc() or
purple_xfer_set_write_fnc() to set up purple backend's auxiliary r/w
functions either.

Only when the functions are not set will libpurple handle the data
transfer on its own using read() and write() on file descriptor we
passed in purple_xfer_start().

See implementation of purple_xfer_read() and purple_xfer_write().