Fix undefined property warning in executeAutoPipeline

Seen in an uncaughtException handler.

```
err_type=uncaughtException, exiting cause=TypeError: Cannot read property 'Symbol(callbacks)' of undefined
    at Immediate.executeAutoPipeline (.../node_modules/ioredis/built/autoPipelining.js:34:31)
    at Shim.applySegment (.../node_modules/newrelic/lib/shim/shim.js:1430:20)
    at Immediate.wrapper (.../node_modules/newrelic/lib/shim/shim.js:2097:17)
    at processImmediate (internal/timers.js:463:21)
```

A similar error was among the errors reported in
redis#1248

I suspect the async nature of the delete call may cause it to be
deleted.

lib/autoPipelining.ts

@@ -27,6 +27,16 @@ function executeAutoPipeline(client, slotKey: string) {
   if (client._runningAutoPipelines.has(slotKey)) {
     return;
   }
+  if (!client._autoPipelines.has(slotKey)) {
+    /* 
+      Rare edge case. Somehow, something has deleted this running autopipeline in an immediate 
+      call to executeAutoPipeline. 
+     
+      Maybe the callback in the pipeline.exec is sometimes called in the same tick,
+      e.g. if redis is disconnected?
+    */
+    return;
+  }
 
   client._runningAutoPipelines.add(slotKey);