Check that the server root directory exists before creating an iterat…

…or to it

There is some room for race condition between this check and iterator creation,
but don't consider it a problem.

test/ftp/client.cpp

@@ -137,11 +137,13 @@ class client : public testing::Test
 
     void TearDown() override
     {
-        // TODO: The server_root_dir_ may not exist.
         /* Clean up the server root directory after each test. */
-        for (const std::filesystem::directory_entry & path : std::filesystem::directory_iterator(server_root_dir_))
+        if (std::filesystem::exists(server_root_dir_))
         {
-            std::filesystem::remove_all(path);
+            for (const std::filesystem::directory_entry & path : std::filesystem::directory_iterator(server_root_dir_))
+            {
+                std::filesystem::remove_all(path);
+            }
         }
     }