Fix handling of invalid entries in the score table.

It may happen that the score table contains references to files that
no longer exist in the file system. The current code fails badly in
this case.

In open_next_audio_file(), if the file corresponding to the next
entry in the score table can not be opened, delete this entry from
the score table and try the next entry.

If the score table becomes empty, send NO_ADMISSIBLE_FILES back to para_server
and make recv_afs_result() switch to stop mode in this case.