kgv1: release reference picture on size change.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit 6c4c27adb6)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/kgv1dec.c

@@ -61,8 +61,11 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, AVPac
     if (av_image_check_size(w, h, 0, avctx))
         return -1;
 
-    if (w != avctx->width || h != avctx->height)
+    if (w != avctx->width || h != avctx->height) {
+        if (c->prev.data[0])
+            avctx->release_buffer(avctx, &c->prev);
         avcodec_set_dimensions(avctx, w, h);
+    }
 
     maxcnt = w * h;