Check for undefined peekCommand queue entry

[jplhomer]

Fixes #26

Problem

In some cases, bad commands (e.g. invalid keys) sent in parallel at a high throughput to a memcache server will result in the error described in #26.

Since this code path runs in an event listener after client initialization and not as a result of a function called in user land, this results in a crashed Node.js process (aka uncaught exception).

Solution

Check retrieve for truthiness before attempting to push a value on it or set .error on it.

This doesn't solve the underlying root/concurrency problem per-se, but it makes the method more resilient to upstream failure.

[CLAassistant]

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