Fix crash in Puma validation initializer

It was using a method before it was defined, which did
not show up in unit tests.

config/initializers/validate_puma.rb

 # frozen_string_literal: true
 
-if Gitlab::Runtime.puma? && ::Puma.cli_config.options[:workers].to_i == 0
-  return if allow_single_mode?
-
-  raise 'Puma is only supported in Cluster-mode: workers > 0'
-end
-
 def allow_single_mode?
   return false if Gitlab.com?
 
   Gitlab::Utils.to_boolean(ENV['PUMA_SKIP_CLUSTER_VALIDATION'])
 end
+
+if Gitlab::Runtime.puma? && ::Puma.cli_config.options[:workers].to_i == 0
+  return if allow_single_mode?
+
+  raise 'Puma is only supported in Cluster-mode: workers > 0'
+end