Avoid segfault on ffmpeg "buffer flush" in mpeg12.c patch by (Wolfram Gloger <wmglo at dent dot med dot uni-muenchen dot de>)

Originally committed as revision 3324 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg12.c

@@ -2769,12 +2769,14 @@ static int mpeg_decode_frame(AVCodecContext *avctx,
     MpegEncContext *s2 = &s->mpeg_enc_ctx;
     dprintf("fill_buffer\n");
 
+    if (buf_size == 0) {
 	/* special case for last picture */
-    if (buf_size == 0 && s2->low_delay==0 && s2->next_picture_ptr) {
+	if (s2->low_delay==0 && s2->next_picture_ptr) {
 	    *picture= *(AVFrame*)s2->next_picture_ptr;
 	    s2->next_picture_ptr= NULL;
 
 	    *data_size = sizeof(AVFrame);
+	}
         return 0;
     }