Benchmark should not print zeros if min_iters >= max_iters

Summary:
Prior to this fix, this benchmark, which internally sets `bm_max_iters` would print out zero timings when `bm_min_iters` was too high:

```
buck2 run @//mode/opt //folly/test:atomic_hash_map_test -- \
  --gtest_filter=X --benchmark=1 --bm_regex=.*_miss --bm_min_iters=1000000
```

Reviewed By: nickbrekhus

Differential Revision: D57396861

fbshipit-source-id: c0b8318be2e572563715197f6546441f57962961

folly/Benchmark.cpp

@@ -899,6 +899,12 @@ void runBenchmarks() {
         "WARNING: Running with constant number of iterations. Results might be jittery.\n");
   }
 
+  if (FLAGS_bm_min_iters >= FLAGS_bm_max_iters) {
+    std::cerr << "WARNING: bm_min_iters > bm_max_iters; increasing the max"
+              << std::endl;
+    FLAGS_bm_max_iters = FLAGS_bm_min_iters + 1;
+  }
+
   checkRunMode();
 
   auto& state = detail::globalBenchmarkState();