gif_read: Set last_name = NULL unconditionally.

With a non-malicious GIF, last_name is set to NULL when a name
extension is followed by an image. Reported in #117, via
Debian, via a KAIST fuzzing program.
file_eofer(Gif_Reader *grr)
{
  int c = getc(grr->f);
  if (c == EOF)
    return 1;
  else {
    ungetc(c, grr->f);
    return 0;
  }
}

file_eofer(Gif_Reader *grr)
{
  int c = getc(grr->f);
  if (c == EOF)
    return 1;
  else {
    ungetc(c, grr->f);
    return 0;
  }
}
