In 81d69f4c (0.2.21-alpha) we added a compile-time check for
a (totally broken) signed size_t.  In 0e597471 (not yet released)
I switched to a better configure-time check, which stored its output
in a different variable.  I didn't change the code which looked at
the output, however.

This bug is not in any released version of Tor, and would not affect
anybody with a working Tor.