ClockEstimate: DivByZero

The clock estimator has a potential division by zero.
Using `iteration + 1` seems also more logical to me for
an average.

Found with coverity in a downstream project.

include/internal/catch_timer.cpp

@@ -40,7 +40,7 @@ namespace Catch {
                 // is terrible and we should move on.
                 // TBD: How to signal that the measured resolution is probably wrong?
                 if (ticks > startTime + 3 * nanosecondsInSecond) {
-                    return sum / i;
+                    return sum / ( i + 1u );
                 }
             }