stopwatch: deal with elapsed == 0 (for whatever reason)

tools/stopwatch.py

@@ -31,7 +31,7 @@ class StopWatch:
         units = ['s', 'ms', 'µs', 'ns']
         unit = 0
         elapsed = self.avg_elapsed(divider)
-        while elapsed < 1:
+        while 0 < elapsed < 1:
             elapsed *= 1000
             unit += 1