Tweak an assert that shouldn't fire either way.

There were however other places where we used to call this
function that might have caused this to fire. Better
safe than sorry now.

src/or/circuitbuild.c

@@ -650,7 +650,7 @@ circuit_build_times_initial_alpha(circuit_build_times_t *cbt,
   // CircBuildTimeout = Xm*((1-0.8))^(-1/a))
   // ln(CircBuildTimeout) = ln(Xm)+ln(((1-0.8)))*(-1/a)
   // -ln(1-0.8)/(ln(CircBuildTimeout)-ln(Xm))=a
-  tor_assert(quantile > 0);
+  tor_assert(quantile >= 0);
   tor_assert(cbt->Xm > 0);
   cbt->alpha = ln(1.0-quantile)/(ln(cbt->Xm)-ln(timeout_ms));
   tor_assert(cbt->alpha > 0);