eacmv: clear references on frame dimensions change

Fixes invalid reads.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/eacmv.c

@@ -141,6 +141,12 @@ static int cmv_process_header(CmvContext *s, const uint8_t *buf, const uint8_t *
     s->width  = AV_RL16(&buf[4]);
     s->height = AV_RL16(&buf[6]);
 
+    if (s->width  != s->avctx->width ||
+        s->height != s->avctx->height) {
+        av_frame_unref(s->last_frame);
+        av_frame_unref(s->last2_frame);
+    }
+
     ret = ff_set_dimensions(s->avctx, s->width, s->height);
     if (ret < 0)
         return ret;