Prevent Memcached undefined error

[bwolfe]

I am getting these errors:

TypeError: Cannot read property 'split' of undefined\n at Client.commandWrapper (/usr/app/node_modules/newrelic/lib/instrumentation/memcached.js:51:51)

on this line of code: mycache.del(key)

The del() call was after a key (probably) timed out or potentially removed by another pod. I cannot replicate this in dev/staging though! >:O

I attempted to write integration tests, but could not get it to fail in this process:

1. Deleting a key after a delete
2. Deleting a key after previous key timed out
3. Deleting a key after end() entire cache.

[NatalieWolfe]

Thanks for this fix, Ben! I'll merge it into our internal repo to make sure it gets released ASAP.