One last underflow in rend_cache tests

We found two instances of this already in our tests: #40099 (closed) and #40125 (closed). In response, I tried running every one of our tests on its own, to see if there are other cases. It looks like we have one last failure:

rend_cache/free_all: Sep 17 14:00:06.862 [warn] rend_cache_decrement_allocation(): Bug: 
Underflow in rend_cache_decrement_allocation (on Tor 0.4.5.0-alpha-dev 404c224c711714f0)

Edited Sep 17, 2020 by Nick Mathewson

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