fix potential buffer over-read

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@10966 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/mpeg.c

@@ -121,7 +121,7 @@ static int mpegps_read_header(AVFormatContext *s,
     s->ctx_flags |= AVFMTCTX_NOHEADER;
 
     get_buffer(&s->pb, buffer, sizeof(buffer));
-    if ((p=memchr(buffer, 'S', sizeof(buffer))))
+    if ((p=memchr(buffer, 'S', sizeof(buffer)-5)))
         if (!memcmp(p, "Sofdec", 6))
             m->sofdec = 1;
     url_fseek(&s->pb, -(offset_t)sizeof(buffer), SEEK_CUR);