as/ec2: ec2.go; 16 LoC

Found a possible issue in [as/ec2](https://www.github.com/as/ec2) at [ec2.go](https://github.com/as/ec2/blob/b787b7f9bdcb7f306337cd1870d8104ec14bbc9d/ec2.go#L64-L79)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first issue it finds, so please do not limit your consideration to the contents of the below message.

> reference to sn was used in a composite literal at line 73

[Click here to see the code in its original context.](https://github.com/as/ec2/blob/b787b7f9bdcb7f306337cd1870d8104ec14bbc9d/ec2.go#L64-L79)

Click here to show the 16 line(s) of Go which triggered the analyzer.

```go
for _, sn := range strings.Split(*subnets, ",") {
x := strings.IndexAny(sn, "@")
var zone *string
if x != -1 {
z := sn[x+1:]
zone = &z
sn = sn[:x]
}
res, err := svc.CreateSubnet(&ec2.CreateSubnetInput{
CidrBlock: &sn,
AvailabilityZone: zone,
VpcId: vpc.VpcId,
})
no(err)
fmt.Printf("subnet: %s: %s\n", res)
}

```

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See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: b787b7f9bdcb7f306337cd1870d8104ec14bbc9d