Make sure to check for empty extension before accessing front. (#1764)

The documentation for std::string::front() says that calling it
on an empty string is undefined behavior.  It actually seems to
work on all platforms except Windows Debug, where it throws an
error.  Make sure to check for empty first.

We also notice that there is no reason to check the
existing_sub_namespace for empty; the rest of the code handles
that fine.  So remove that check.

Finally, we add an anonymous namespace so that these functions do
not pollute the global namespace.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

rclcpp/src/rclcpp/node.cpp

@@ -47,25 +47,28 @@ using rclcpp::Node;
 using rclcpp::NodeOptions;
 using rclcpp::exceptions::throw_from_rcl_error;
 
+namespace
+{
+
 RCLCPP_LOCAL
 std::string
 extend_sub_namespace(const std::string & existing_sub_namespace, const std::string & extension)
 {
   // Assumption is that the existing_sub_namespace does not need checking
   // because it would be checked already when it was set with this function.
 
-  // check if the new sub-namespace extension is absolute
-  if (extension.front() == '/') {
+  if (extension.empty()) {
     throw rclcpp::exceptions::NameValidationError(
             "sub_namespace",
             extension.c_str(),
-            "a sub-namespace should not have a leading /",
+            "sub-nodes should not extend nodes by an empty sub-namespace",
             0);
-  } else if (existing_sub_namespace.empty() && extension.empty()) {
+  } else if (extension.front() == '/') {
+    // check if the new sub-namespace extension is absolute
     throw rclcpp::exceptions::NameValidationError(
             "sub_namespace",
             extension.c_str(),
-            "sub-nodes should not extend nodes by an empty sub-namespace",
+            "a sub-namespace should not have a leading /",
             0);
   }
 
@@ -76,7 +79,7 @@ extend_sub_namespace(const std::string & existing_sub_namespace, const std::stri
     new_sub_namespace = existing_sub_namespace + "/" + extension;
   }
 
-  // remove any trailing `/` so that new extensions do no result in `//`
+  // remove any trailing `/` so that new extensions do not result in `//`
   if (new_sub_namespace.back() == '/') {
     new_sub_namespace = new_sub_namespace.substr(0, new_sub_namespace.size() - 1);
   }
@@ -104,6 +107,8 @@ create_effective_namespace(const std::string & node_namespace, const std::string
   }
 }
 
+}  // namespace
+
 Node::Node(
   const std::string & node_name,
   const NodeOptions & options)