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.

src/gifread.c

@@ -900,6 +900,7 @@ read_gif(Gif_Reader *grr, int read_flags,
   Gif_DeleteArray(gfc.suffix);
   Gif_DeleteArray(gfc.length);
   gfc.gfi = 0;
+  last_name = 0;
 
   if (gfs)
     gfs->errors = gfc.errors[1];