Improve error handling of CppServiceHandler

[andocz]

Sometimes a segfault happens when opening "This calls" in the info tree. For example at tinyxml2::XMLDocument::SetError.

I fixed this by adding a check against the query matching nothing, like how it's already done with other queries in the code. The query is empty if the AST node has no corresponding CPP entity. In TinyXML2's case this happens with __va_list_tag::__va_list_tag(). I assume this is an anomaly, so I added a warning log message.

I also updated an extremely outdated comment.