10l (segfault fix)

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@4341 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/aviobuf.c

@@ -272,7 +272,8 @@ static void fill_buffer(ByteIOContext *s)
         return;
 
     if(s->update_checksum){
-        s->checksum= s->update_checksum(s->checksum, s->checksum_ptr, s->buf_end - s->checksum_ptr);
+        if(s->buf_end > s->checksum_ptr)
+            s->checksum= s->update_checksum(s->checksum, s->checksum_ptr, s->buf_end - s->checksum_ptr);
         s->checksum_ptr= s->buffer;
     }