Avoid null ptr ref for for yaz_poll errors.

Avoid null ptr ref for for yaz_poll errors.

When yaz_poll returns error (-1) and errno is not EINTR the code
previously assumed seshigh IOCHAN (client_link) and called cs_close..
In the mother process, however, the IOCHAN data is not this type.
Code now prints a warning and resumes processing and does not use
iochan_getdata.