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
