string data shares a buffer with the binary string length field. size calculations for string read buffers always include the length field; sufficiently large length fields can cause these calculations to wrap. a malicious nar could use this for OOB writes in the daemon (as root). since we use strings only as tags for archive members and for symlinks with their OS-dependent length limits we can simply limit string size. 1 MiB should be sufficient for all symlinks, and tags are always tiny. Change-Id: I89fb05f73c1dbeda45d91244aba4cd526a3d83e1