SAML redirect to login page fails if AM installed into the root context

Bug description

If AM is installed into the container root context, then when you hit /idpssoinit to being a SAML flow the redirect to the AM login UI fails because an incorrect redirect is generated.

How to reproduce the issue

1. Install AM into the container root context
2. Configure SAML with a hosted IDP and remote SP
3. Start an SSO flow using /idpssoinit (or idpSSOInit.jsp)

Expected behaviour

User is redirected to /UI/Login and then to the XUI to login.

Current behaviour

User is redirected to /idpssoinit/UI/Login or /saml2/UI/Login if using the JSP directly.

Work around

Explicitly configure the AuthUrl in the IDP settings (under Assertion Processing) to the full login URL.

Code analysis

String uri = request.getRequestURI();
String deploymentURI = uri;
int firstSlashIndex = uri.indexOf("/");
int secondSlashIndex = uri.indexOf("/", firstSlashIndex + 1);
if (secondSlashIndex != -1) {
    deploymentURI = uri.substring(0, secondSlashIndex);
}

This code assumes that there is a non-empty context path between the first two slashes in the URL. When deployed into the context root this is not true as the context path is empty.