cloud/awscloud: fix nil pointer dereference

[croissanne]

When the cleanup function gets called, there's a chance the Instnace field isn't populated yet, so store the instance ID separately and wait for it to be terminated in case it's present.

The error would produce the following trace:

goroutine 1 [running]:
...
main.(*OSBuildJobImpl).Run.func1()
    osbuild/osbuild-composer/cmd/osbuild-worker/jobimpl-osbuild.go:404 +0xc5
panic({0x55e2a76a1e40?, 0x55e2a906d2f0?})
    /usr/lib/golang/src/runtime/panic.go:920 +0x270
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).deleteFleetIfExists(0xc000faa840, 0xc0012718c0)
    osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:441 +0x175
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).TerminateSecureInstance(0x55e2a90825e0?, 0x2?)
    osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:192 +0x1d
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).RunSecureInstance.func1()
    osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:75 +0x69
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).RunSecureInstance(0xc000faa840, {0xc000afeade, 0x10}, {0x0, 0x0}, {0x0, 0x0}, {0xc001120f30, 0x24})
    osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:169 +0x12a7
...

This pull request includes:

• adequate testing for the new functionality or fixed issue
• adequate documentation informing people about the change such as
  • submit a PR for the READMEs listed here
  • submit a PR for the osbuild.org website repository if this PR changed any behavior not covered by the automatically updated READMEs

[schuellerf]

Although a test is missing, this probably should go towards production to avoid crashes…

[croissanne]

Although a test is missing, this probably should go towards production to avoid crashes…

The integration test tests the golden path, but unit testing is missing. AWS does provide things like https://docs.aws.amazon.com/sdk-for-go/api/service/ec2/ec2iface/ to help mock, would be worth looking into.