avcodec/snow: fix null pointer dereference in cleanup after allocation failure

Fixes: snowf.avi
Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/snow.c

@@ -703,7 +703,7 @@ av_cold void ff_snow_common_end(SnowContext *s)
     for(i=0; i<MAX_REF_FRAMES; i++){
         av_freep(&s->ref_mvs[i]);
         av_freep(&s->ref_scores[i]);
-        if(s->last_picture[i]->data[0]) {
+        if(s->last_picture[i] && s->last_picture[i]->data[0]) {
             av_assert0(s->last_picture[i]->data[0] != s->current_picture->data[0]);
         }
         av_frame_free(&s->last_picture[i]);