Sanity check FASTA files.

def sanity_check_fasta(handle):
    """Sanity check FASTA files."""
    header_found = False
    for line in handle:
        if line.startswith('>'):
            header_found = True
            break

    handle.seek(0)

    if header_found:
        return handle

    fake_header_line = ">DUMMY"
    new_handle = StringIO()
    new_handle.write("%s\n" % fake_header_line)
    new_handle.write(handle.read())
    new_handle.seek(0)
    return new_handle