avcodec/mpegvideo: fix null pointer dereference

Fixes: 111342.ogm with ffplay
Found-by: Dale Curtis <dalecurtis@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.c

@@ -931,7 +931,7 @@ int ff_mpeg_update_thread_context(AVCodecContext *dst,
 #define UPDATE_PICTURE(pic)\
 do {\
     ff_mpeg_unref_picture(s, &s->pic);\
-    if (s1->pic.f->buf[0])\
+    if (s1->pic.f && s1->pic.f->buf[0])\
         ret = ff_mpeg_ref_picture(s, &s->pic, &s1->pic);\
     else\
         ret = update_picture_tables(&s->pic, &s1->pic);\