util: plug a file descriptor leak

It's possible that a file descriptor would leak if fdopen() failed for
any reason. Found by Coverity.

src/util.c

@@ -388,8 +388,14 @@ netplan_util_dump_yaml_subtree(const char* prefix, int input_fd, int output_fd,
 cleanup:
     if (input)
         fclose(input);
+    else
+        close(in_dup);
+
     if (output)
         fclose(output);
+    else
+        close(out_dup);
+
     if (yaml_path)
         g_strfreev(yaml_path);
     return ret;

tests/test_libnetplan.py

@@ -605,13 +605,25 @@ def test_create_yaml_patch_bad_syntax(self):
             patchfile.seek(0, io.SEEK_END)
             self.assertEqual(patchfile.tell(), 0)
 
-    def test_dump_yaml_subtree_bad_file_perms(self):
+    def test_dump_yaml_subtree_bad_input_file_perms(self):
         input_file = os.path.join(self.workdir.name, 'input.yaml')
         with open(input_file, "w") as f, tempfile.TemporaryFile() as output:
             with self.assertRaises(libnetplan.NetplanFileException) as context:
                 libnetplan.dump_yaml_subtree('network', f, output)
         self.assertIn('Invalid argument', str(context.exception))
 
+    def test_dump_yaml_subtree_bad_output_file_perms(self):
+        input_file = os.path.join(self.workdir.name, 'input.yaml')
+        output_file = os.path.join(self.workdir.name, 'output.yaml')
+        with open(input_file, 'w') as input, open(output_file, 'w') as output:
+            input.write('network: {}')
+            output.write('')
+
+        with open(input_file, "r") as f, open(output_file, 'r') as output:
+            with self.assertRaises(libnetplan.NetplanFileException) as context:
+                libnetplan.dump_yaml_subtree('network', f, output)
+        self.assertIn('Invalid argument', str(context.exception))
+
     def test_dump_yaml_subtree_bad_yaml_outside(self):
         input_file = os.path.join(self.workdir.name, 'input.yaml')
         with open(input_file, "w+") as f, tempfile.TemporaryFile() as output: