stream: improve EOF handling in seeks

stream: improve EOF handling in seeks

Reset stream 'eof' flag when a seek succeeds, and allow seeking to a
position at or past EOF (in the sense that the seek succeeds and
stream_tell() then returns that position).

This fixes at least some demuxer problems where an attempt to read
the index from the end of an incomplete file would set the 'eof' flag
and cause subsequent reads to fail, even if failure to read the index
would otherwise be nonfatal and demuxing could continue after seeking
back.

Partially based on a patch from Laurent <laurent.aml@gmail.com>.