Fix bug in memory quota fuzzer (#27670)

* flush more thoroughly

* add reproducer

* fixes

test/core/resource_quota/memory_quota_fuzzer.cc

@@ -45,9 +45,13 @@ class Fuzzer {
   void Run(const memory_quota_fuzzer::Msg& msg) {
     grpc_core::ExecCtx exec_ctx;
     RunMsg(msg);
-    memory_quotas_.clear();
-    memory_allocators_.clear();
-    allocations_.clear();
+    do {
+      memory_quotas_.clear();
+      memory_allocators_.clear();
+      allocations_.clear();
+      exec_ctx.Flush();
+    } while (!memory_quotas_.empty() || !memory_allocators_.empty() ||
+             !allocations_.empty());
   }
 
  private:

test/core/resource_quota/memory_quota_fuzzer_corpus/nullchk

@@ -0,0 +1,21 @@
+actions {
+  create_quota {}
+}
+actions {
+  create_allocator {}
+}
+actions {
+  post_reclaimer {
+    msg {
+      actions {
+        create_quota {}
+      }
+      actions {
+        create_allocator {}
+      }
+      actions {
+        post_reclaimer {}
+      }
+    }
+  }
+}