segfault fix

Originally committed as revision 2434 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -585,9 +585,11 @@ void MPV_common_end(MpegEncContext *s)
     av_freep(&s->input_picture);
     av_freep(&s->reordered_input_picture);
 
+    if(s->picture){
         for(i=0; i<MAX_PICTURE_COUNT; i++){
             free_picture(s, &s->picture[i]);
         }
+    }
     av_freep(&s->picture);
     avcodec_default_free_buffers(s->avctx);
     s->context_initialized = 0;