GUACAMOLE-234: Only try LDAP bind if user authenticated with LDAP.

extensions/guacamole-auth-ldap/src/main/java/org/apache/guacamole/auth/ldap/AuthenticationProviderService.java

@@ -300,6 +300,7 @@ public class AuthenticationProviderService {
 
         // Bind using credentials associated with AuthenticatedUser
         Credentials credentials = authenticatedUser.getCredentials();
+        if (authenticatedUser instanceof LDAPAuthenticatedUser) {
             Dn bindDn = ((LDAPAuthenticatedUser) authenticatedUser).getBindDn();
             LdapNetworkConnection ldapConnection = ldapService.bindAs(bindDn, credentials.getPassword());
 
@@ -316,6 +317,8 @@ public class AuthenticationProviderService {
             finally {
                 ldapService.disconnect(ldapConnection);
             }
+        }
+        return null;
 
     }