Check for nil before closing Uniquefile

On Windows, we by default assign stdout for the child process to a file:

    stdout = Puppet::FileSystem::Uniquefile.new('puppet')

However, if TMPDIR, TEMP or TMP env vars refer to a non-existent directory, then
`Uniquefile.new` will raise, leaving `stdout` set to nil, so the later call to
`stdout.close` raised NoMethodError.

So check for nil before closing. We don't have this issue on posix, because
it uses an IO pipe instead of Uniquefile.

Fixes #9385

Author: joshcooper

lib/puppet/util/execution.rb

@@ -323,7 +323,7 @@ def self.execute(command, options = NoOptionsSpecified)
       unless options[:squelch]
         # if we opened a pipe, we need to clean it up.
         reader.close if reader
-        stdout.close! if Puppet::Util::Platform.windows?
+        stdout.close! if stdout && Puppet::Util::Platform.windows?
       end
     end
 

spec/unit/util/execution_spec.rb

@@ -834,6 +834,15 @@ def expect_cwd_to_be(cwd)
 
           Puppet::Util::Execution.execute('test command')
         end
+
+        it "should raise if it fails to create a Uniquefile for stdout" do
+          allow(Puppet::FileSystem::Uniquefile).to receive(:new)
+            .and_raise(Errno::ENOENT, 'C:\Users\ADMINI~1\AppData\Local\Temp\doesnotexist')
+
+          expect {
+            Puppet::Util::Execution.execute('test command')
+          }.to raise_error(Errno::ENOENT, 'No such file or directory - C:\Users\ADMINI~1\AppData\Local\Temp\doesnotexist')
+        end
       end
 
       it "should raise an error if failonfail is true and the child failed" do