Malformed role causes panic

[preflightsiren]

We observed an issue with KIAM 4.2 today in our engineering environment when someone manually edited the IAM role for a deployment and pasted in a malformed ARN (incorrect number of :'s)

I've attached a quick test to help prove the error, and I have a branch locally with a reasonable fix IMO.

pkg/aws/sts/arn_resolver_test.go

func TestMalformedInstanceArn(t *testing.T) {
	defer func() {
		r := recover()
		if (r != nil) {
			t.Errorf("Unexpected panic from resolver.Resolve()")
		}
	}()
	resolver := DefaultResolver("arn:aws:iam::account-id:role/")
	_, _ = resolver.Resolve("arn:aws:iam:account-id:role/my-role")
}

❯ go test ./pkg/aws/sts -race
--- FAIL: TestMalformedInstanceArn (0.00s)
    arn_resolver_test.go:115: Unexpected panic from resolver.Resolve()
time="2022-10-25T13:35:23+11:00" level=info msg="requested new credentials" credentials.access.key= credentials.expiration= credentials.role="arn:account:role"
time="2022-10-25T13:35:23+11:00" level=info msg="requested new credentials" credentials.access.key= credentials.expiration= credentials.role="arn:account:role" credentials.session-name=testing
time="2022-10-25T13:35:23+11:00" level=info msg="requested new credentials" credentials.access.key= credentials.expiration= credentials.role="arn:account:role" credentials.session-name="testing@#&-test%"
time="2022-10-25T13:35:23+11:00" level=info msg="requested new credentials" credentials.access.key= credentials.expiration= credentials.role="arn:account:role" credentials.session-name=****
time="2022-10-25T13:35:23+11:00" level=info msg="requested new credentials" credentials.access.key= credentials.expiration= credentials.external-id="******" credentials.role="arn:account:role"
FAIL
FAIL    github.com/uswitch/kiam/pkg/aws/sts     2.347s
FAIL

Given the high liklihood of user error, and the high impact (panic), I would hope we can merge and release a patch version,