Assert there's no active scheduler before freezing

runtime/src/bank.rs

@@ -3236,6 +3236,10 @@ impl Bank {
         // committed before this write lock can be obtained here.
         let mut hash = self.hash.write().unwrap();
         if *hash == Hash::default() {
+            // there should be no active scheduler at this point, which
+            // might be actively mutating bank state...
+            assert!(!self.with_scheduler());
+
             // finish up any deferred changes to account state
             self.collect_rent_eagerly();
             self.collect_fees();