Eof handling

When performing 

    fatt extract --file seqlist a.fasta

after 

    fatt index a.fasta

if the seqlist contains the last element of the a.fasta file, retrieval of any record after the specific entry were failing. To my understanding, the situation is caused because no code clear the eof bit which was set when reading the last entry.  The attached patch clears every time before seekg().  This could be overkill and clearing after hitting the eof might be better, though I did not analyze in depth if it can be safely done.  I am not sure if clearing eof just after finding eof is safe, but clearing before seek should be safe albeit it may not be best in terms of performance, and the performance penalty should not be too large.