bench: Fix subtle counting issue when rescaling iteration count

Make sure that the count is a zero modulo the new mask before
scaling, otherwise the next time until a measure triggers
will take only 1/2 as long as accounted for. This caused
the 'min time' to be potentially off by as much as 100%.

src/bench/bench.cpp

@@ -64,8 +64,11 @@ bool State::KeepRunning()
           return true;
         }
         if (elapsed*16 < maxElapsed) {
-          countMask = ((countMask<<1)|1) & ((1LL<<60)-1);
-          countMaskInv = 1./(countMask+1);
+          uint64_t newCountMask = ((countMask<<1)|1) & ((1LL<<60)-1);
+          if ((count & newCountMask)==0) {
+              countMask = newCountMask;
+              countMaskInv = 1./(countMask+1);
+          }
         }
     }
     lastTime = now;