Use proper array lenght

See PR #1233 and discussion therein.
(cherry picked from commit b8c3f61c)

core/src/main/java/jenkins/security/LastGrantedAuthoritiesProperty.java

@@ -50,7 +50,7 @@ public class LastGrantedAuthoritiesProperty extends UserProperty {
         GrantedAuthority[] r = new GrantedAuthority[roles==null ? 1 : roles.length+1];
         r[0] = SecurityRealm.AUTHENTICATED_AUTHORITY;
         if (roles != null) {
-            for (int i = 1; i < roles.length; i++) {
+            for (int i = 1; i < r.length; i++) {
                 r[i] = new GrantedAuthorityImpl(roles[i - 1]);
             }
         }