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.
gifgetunsigned(Gif_Reader *grr)
{
  uint8_t one = gifgetbyte(grr);
  uint8_t two = gifgetbyte(grr);
  return one | (two << 8);
}

gifgetunsigned(Gif_Reader *grr)
{
  uint8_t one = gifgetbyte(grr);
  uint8_t two = gifgetbyte(grr);
  return one | (two << 8);
}
