hidserv-stats file name not handled by our sandbox

This bug is self explainable:

Oct 15 13:30:56.000 [warn] sandbox_intern_string(): Bug: No interned sandbox parameter found for /chutney/net/nodes/000a/stats/hidserv-stats (on Tor 0.2.8.0-alpha-dev 9c39a11f866223f0)

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