#96 Fixes memory leak due to Oct 1st regression in processItems

[kevburnsjr]

See #96

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I ran the proof with two different versions of ristretto and got 2 different results.
This is definitely a regression.

Found the offending commit
d963fa2

$ go get github.com/dgraph-io/ristretto@d963fa241740c4012b003362a7602e6ae764b104
go: finding github.com/dgraph-io/ristretto d963fa241740c4012b003362a7602e6ae764b104
go: downloading github.com/dgraph-io/ristretto v0.0.0-20191001142246-d963fa241740
go: extracting github.com/dgraph-io/ristretto v0.0.0-20191001142246-d963fa241740

$ go run ristretto-bug.go
time: 1s alloc: 1034.71 MiB hits: 75 misses: 3747
time: 2s alloc: 1043.73 MiB hits: 117 misses: 4378
time: 3s alloc: 1050.75 MiB hits: 109 misses: 4374
time: 4s alloc: 1076.77 MiB hits: 138 misses: 4339
time: 5s alloc: 1126.79 MiB hits: 178 misses: 4337

$ go get github.com/dgraph-io/ristretto@7028ca5adaaeebef6f8498040b8a77189a4387b2
go: finding github.com/dgraph-io/ristretto 7028ca5adaaeebef6f8498040b8a77189a4387b2
go: downloading github.com/dgraph-io/ristretto v0.0.0-20190930223733-7028ca5adaae
go: extracting github.com/dgraph-io/ristretto v0.0.0-20190930223733-7028ca5adaae

$ go run ristretto-bug.go
time: 1s alloc: 1040.71 MiB hits: 78 misses: 3807
time: 2s alloc: 1031.72 MiB hits: 105 misses: 4403
time: 3s alloc: 1030.73 MiB hits: 110 misses: 4373
time: 4s alloc: 1036.74 MiB hits: 114 misses: 4335
time: 5s alloc: 1029.75 MiB hits: 107 misses: 4370

Reading the diff I saw a logic change that looked like an error since policy.Add can, in some cases, return victims when added is false.
https://github.com/dgraph-io/ristretto/blob/master/policy.go#L168

// Before Oct 1st

victims, added := c.policy.Add(item.key, item.cost)
if added {
	// ...
}
for _, victim := range victims {
	// ...
}

// After Oct 1st commit

if victims, added := c.policy.Add(item.key, item.cost); added {
	// ...
	for _, victim := range victims {
		// ...
	}
}

This commit extracts the for loop from the if statement, fixing the memory leak.

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This change is 

[karlmcguire]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jarifibrahim and @manishrjain)

[karlmcguire]

Thanks @kevburnsjr.