stopwatch: Fix buffer underflow when computing percentiles.

UB Sanitizer report:
  lib/stopwatch.c:119:22: runtime error: index 18446744073709551615 out of
                          bounds for type 'long long unsigned int [50]'
      #0 0x698358 in calc_percentile lib/stopwatch.c:119
      #1 0x69ada1 in add_sample lib/stopwatch.c:231
      #2 0x69c086 in stopwatch_end_sample_protected lib/stopwatch.c:386
      #3 0x69c522 in stopwatch_thread lib/stopwatch.c:441
      #4 0x684bae in ovsthread_wrapper lib/ovs-thread.c:383
      #5 0x7f042838b298 in start_thread (/lib64/libpthread.so.0+0x9298)
      #6 0x7f04277f2352 in clone (/lib64/libc.so.6+0x100352)

Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Acked-by: Paolo Valerio <pvalerio@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/stopwatch.c

@@ -114,7 +114,6 @@ static void
 calc_percentile(unsigned long long n_samples, struct percentile *pctl,
                 unsigned long long new_sample)
 {
-
     if (n_samples < P_SQUARE_MIN) {
         pctl->samples[n_samples - 1] = new_sample;
     }
@@ -228,13 +227,12 @@ add_sample(struct stopwatch *sw, unsigned long long new_sample)
         sw->min = new_sample;
     }
 
-    calc_percentile(sw->n_samples, &sw->pctl, new_sample);
-
     if (sw->n_samples++ == 0) {
         sw->short_term.average = sw->long_term.average = new_sample;
         return;
     }
 
+    calc_percentile(sw->n_samples, &sw->pctl, new_sample);
     calc_average(&sw->short_term, new_sample);
     calc_average(&sw->long_term, new_sample);
 }