Minor readability change to TagStore.pullImage

This is almost entirely a whitespace change from:

```go
for foo {
    if !condition {
        <large amount of code>
    }
    <done>
}
```

To:

```go
for foo {
    if condition {
        <done>
        continue
    }
    <large amount of code>
    <done>
}
```

It's a small stepwise change which improves the readability of the code.
It makes it so you don't have to skip over `<large amount of code>` to
see the `condition == true` outcome, improving locality, and removes a
layer of indentation to `<large amount of code>`.

I'm making this change in advance of other proposed changes I have inbound for this code relating to cancelling builds and pulls.

/cc @shykes (original author)

Docker-DCO-1.1-Signed-off-by: Peter Waller <p@pwaller.net> (github: pwaller)