istream_nfs: move pend_read check from _schedule_read() to _schedule_read_or_eof()
istream_nfs: move pend_read check from _schedule_read() to _schedule_read_or_eof()

Fixes assertion failure.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy