gifdec: reset previous Graphic Control Extension disposal type

This fixes out of array accesses.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/gifdec.c

@@ -461,6 +461,7 @@ static int gif_decode_frame(AVCodecContext *avctx, void *data, int *got_frame, A
 
     if (s->keyframe) {
         s->keyframe_ok = 0;
+        s->gce_prev_disposal = GCE_DISPOSAL_NONE;
         if ((ret = gif_read_header1(s)) < 0)
             return ret;