Fix undefined property Error in executeAutoPipeline

[TysonAndre]

Seen in an uncaughtException handler with ioredis 4.27.7

The application having this issue has a constant&high volume of redis calls, and is using both Redis and Redis.Cluster clients to connect to different redis servers. So it might be related to redis cluster. Or newrelic, but I doubt it, processImmediate is something I'd guess would be from the setImmediate call?

This is happening throughout the day

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
#1248

I suspect the process.exec might call the callback synchronously in the
same tick in some cases, immediately deleting the autopipeline from
running pipelines?

i.e. my guess is that

1. Something calls setImmediate for executeAutoPipeline
2. Something else calls setImmediate
3. The next tick starts
4. immediateHandler calls executeAutoPipeline the first time, and the .exec somehow synchronously happens(same tick) and deletes the key from the Map
5. immediateHandler calls executeAutoPipeline the second time for the same shard key, but because the shardKey was deleted, the fetched value was undefined

EDIT: This seems to have worked - the service remains running and stable with no issues after upgrading to the ioredis release including this fix (4.27.9) (and Cannot read property 'Symbol(callbacks)' of undefined stopped)

[TysonAndre]

This is ready for review. Any comments?

[ioredis-robot]

🎉 This PR is included in version 4.27.9 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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