Check for nil return from forwarded ports cap

Nil may be returned if the forwarded ports cap is called while the
machine is not in a running state.

plugins/communicators/winrm/helper.rb

@@ -58,7 +58,7 @@ module VagrantPlugins
           # ports.
           port = nil
           if machine.provider.capability?(:forwarded_ports)
-            machine.provider.capability(:forwarded_ports).each do |host, guest|
+            Array(machine.provider.capability(:forwarded_ports)).each do |host, guest|
               if guest == machine.config.winrm.guest_port
                 port = host
                 break

test/unit/plugins/communicators/winrm/helper_test.rb

@@ -147,6 +147,15 @@ describe VagrantPlugins::CommunicatorWinRM::Helper do
 
       expect(subject.winrm_port(machine)).to eq(2456)
     end
+
+    it "does not error when the provider capability returns nil result" do
+      machine.config.winrm.port = 22
+      machine.config.winrm.guest_port = 2222
+      machine.config.vm.network "forwarded_port", host: 1234, guest: 2222
+      allow(machine.provider).to receive(:capability).with(:forwarded_ports).and_return(nil)
+
+      expect(subject.winrm_port(machine)).to eq(1234)
+    end
   end
 
   describe ".winrm_info_invalid?" do