Fix uninitialized memory in test (#2498)

When I added in the tests for large messages, I made a mistake and reserved space in the strings, but didn't actually expand it.  Thus, we were writing into uninitialized memory.  Fix this by just using the correct constructor for string, which will allocate and initialize the memory properly.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rclcpp/test/rclcpp/test_publisher_with_type_adapter.cpp

@@ -379,9 +379,7 @@ TEST_F(TestPublisher, test_large_message_unique)
   auto pub = node->create_publisher<StringTypeAdapter>(topic_name, 1);
 
   static constexpr size_t length = 10 * 1024 * 1024;
-  auto message_data = std::make_unique<std::string>();
-  message_data->reserve(length);
-  std::fill(message_data->begin(), message_data->begin() + length, '#');
+  auto message_data = std::make_unique<std::string>(length, '#');
   pub->publish(std::move(message_data));
 }
 
@@ -400,8 +398,6 @@ TEST_F(TestPublisher, test_large_message_constref)
   auto pub = node->create_publisher<StringTypeAdapter>(topic_name, 1);
 
   static constexpr size_t length = 10 * 1024 * 1024;
-  std::string message_data;
-  message_data.reserve(length);
-  std::fill(message_data.begin(), message_data.begin() + length, '#');
+  std::string message_data(length, '#');
   pub->publish(message_data);
 }