vp3: Copy all 3 frames for thread updates.

This fixes a double release of the current frame on deinit.
Fixes CVE-2011-3934

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

libavcodec/vp3.c

@@ -1859,7 +1859,7 @@ static int vp3_update_thread_context(AVCodecContext *dst, const AVCodecContext *
         ||s->width != s1->width
         ||s->height!= s1->height) {
         if (s != s1)
-            copy_fields(s, s1, golden_frame, current_frame);
+            copy_fields(s, s1, golden_frame, keyframe);
         return -1;
     }