runtime: fix TOCTOU issue when decreasing num_idle_threads (#7918)

Author: ADD-SP

tokio/src/runtime/blocking/pool.rs

@@ -568,13 +568,13 @@ impl Inner {
         // Thread exit
         self.metrics.dec_num_threads();
 
-        // num_idle should now be tracked exactly, panic
+        // `num_idle_threads` should now be tracked exactly, panic
         // with a descriptive message if it is not the
         // case.
         let prev_idle = self.metrics.dec_num_idle_threads();
-        assert!(
-            prev_idle >= self.metrics.num_idle_threads(),
-            "num_idle_threads underflowed on thread exit"
+        assert_ne!(
+            prev_idle, 0,
+            "`num_idle_threads` underflowed on thread exit"
         );
 
         if shared.shutdown && self.metrics.num_threads() == 0 {