Fix db_bench memory use after free (detected by clang_analyze)

Summary: Fix using `arg[i].thread` after deleting it

Test Plan: run clang_analyze

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D63171

tools/db_bench_tool.cc

@@ -2460,11 +2460,6 @@ class Benchmark {
     }
     shared.mu.Unlock();
 
-    for (int i = 0; i < n; i++) {
-      delete arg[i].thread;
-    }
-    delete[] arg;
-
     // Stats for some threads can be excluded.
     Stats merge_stats;
     for (int i = 0; i < n; i++) {
@@ -2472,6 +2467,11 @@ class Benchmark {
     }
     merge_stats.Report(name);
 
+    for (int i = 0; i < n; i++) {
+      delete arg[i].thread;
+    }
+    delete[] arg;
+
     return merge_stats;
   }