fix postfix::satellite class

i got this error using postfix:📡

  Error: Could not retrieve catalog from remote server: Error 400 on
  SERVER: Cannot reassign variable valid_fqdn at
  /etc/puppet/modules/postfix/manifests/satellite.pp:32 …

reassigning a variable within a given scope doesn't work in puppet
see https://docs.puppetlabs.com/puppet/latest/reference/lang_variables.html#no-reassignment

i still think overriding $::fqdn is not a good idea, but for now,
i just want to get this class working again.

manifests/satellite.pp

@@ -21,16 +21,16 @@
 #     }
 #   }
 #
+
 class postfix::satellite(
   $relayhost           = '',
-  $valid_fqdn          = '',
+  $valid_fqdn          = $::fqdn,
   $root_mail_recipient = ''
 ) {
 
-  # If $valid_fqdn exists, use it to override $fqdn
-  case $valid_fqdn {
-    "":      { $valid_fqdn = $::fqdn }
-    default: { $fqdn = "${valid_fqdn}" }
+  # If $valid_fqdn is provided, use it to override $fqdn
+  if $valid_fqdn != $::fdqn {
+    $fqdn = $valid_fqdn
   }
 
   class { 'postfix':