[sleep] Fix use after free discovered by fuzzer (#30542)

* minimized crash

* [sleep] Fix use after free discovered by fuzzer

test/core/end2end/fuzzers/api_fuzzer_corpus/sleep-crash

@@ -0,0 +1,32 @@
+actions {
+  create_server {}
+}
+actions {
+  create_channel {
+  target:
+    "unixpepPthtGK9223372036854775807"
+    channel_args{key : "grpc.client_idle_timeout_ms" i : 4297} channel_actions {
+    }
+  }
+}
+actions {
+  create_call {
+    method {
+    value:
+      "w"
+    }
+  }
+}
+actions {
+  ping {}
+}
+actions {
+  request_call {}
+}
+actions {
+  queue_batch {
+    operations {
+      send_initial_metadata {}
+    }
+  }
+}

test/core/event_engine/fuzzing_event_engine/fuzzing_event_engine.cc

@@ -121,8 +121,11 @@ void FuzzingEventEngine::Tick() {
     ++current_tick_;
     // Find newly expired timers.
     while (!tasks_by_time_.empty() && tasks_by_time_.begin()->first <= now_) {
-      tasks_by_id_.erase(tasks_by_time_.begin()->second->id);
+      auto& task = *tasks_by_time_.begin()->second;
-      to_run.push_back(std::move(tasks_by_time_.begin()->second->closure));
+      tasks_by_id_.erase(task.id);
+      if (task.closure != nullptr) {
+        to_run.push_back(std::move(task.closure));
+      }
       tasks_by_time_.erase(tasks_by_time_.begin());
     }
   }
@@ -197,10 +200,10 @@ bool FuzzingEventEngine::Cancel(TaskHandle handle) {
   if (it == tasks_by_id_.end()) {
     return false;
   }
-  if (it->second == nullptr) {
+  if (it->second->closure == nullptr) {
     return false;
   }
-  it->second = nullptr;
+  it->second->closure = nullptr;
   return true;
 }