snapcore/snapd: gadget/update.go; 13 LoC

Found a possible issue in [snapcore/snapd](https://www.github.com/snapcore/snapd) at [gadget/update.go](https://github.com/snapcore/snapd/blob/0e619da461982e6564a93098660087cc765d6327/gadget/update.go#L315-L327)

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.

>

[Click here to see the code in its original context.](https://github.com/snapcore/snapd/blob/0e619da461982e6564a93098660087cc765d6327/gadget/update.go#L315-L327)

Click here to show the 13 line(s) of Go which triggered the analyzer.

```go
for j, oldStruct := range oldVol.LaidOutStructure {
newStruct := newVol.LaidOutStructure[j]
// update only when new edition is higher than the old one; boot
// assets are assumed to be backwards compatible, once deployed
// are not rolled back or replaced unless a higher edition is
// available
if policy(&oldStruct, &newStruct) {
updates = append(updates, updatePair{
from: &oldVol.LaidOutStructure[j],
to: &newVol.LaidOutStructure[j],
})
}
}

```

Click here to show extra information the analyzer produced.

```
No path was found through the callgraph that could lead to a function which writes a pointer argument.

No path was found through the callgraph that could lead to a function which passes a pointer to third-party code.

root signature {policy 2} was not found in the callgraph; reference was passed directly to third-party code
```

Leave a reaction on this issue to contribute to the project by classifying this instance as a **Bug** :-1:, **Mitigated** :+1:, or **Desirable Behavior** :rocket:
See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: 0e619da461982e6564a93098660087cc765d6327