Look at the right variable when warning about signed size_t.
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.