Nowadays, I have some time that I can easily put to good use by helping developers to write code, to debug and to improve already existing code.

At the moment, my preference go for applications intended for the CLI, although such code can easily be added to gui based programs. I don't want to use any QT based libraries as I prefer a simple API. I can use C and C++ but I prefer C++.

Am I only dreaming the impossible?

Contributing with writing code is indeed possible. I would recommend to start with choosing a program where you would like to contribute and have a look at open bug reports. Maybe you can fix a few and provide patches. I would think that this is the easiest way to become familiar with the already existing code-base. Of course you also can start with implementing features and commit those, but you have to be aware if those features are actually something that fits to the philosophy of the project at hand. It may also help to search contact to the developers using mailing lists and/or IRC channels.
But still, after all, most open source projects are more or less meritocracies: If you write it, it is stable and fits to the project it is likely to get your changes integrated.

Where can I find "open bug reports" that are relevant to the cause of GNU/Linux?

I tried google.com, but it gave me a list of irrelevant hits.

Well, many here are familiar with bug reports and where to find them. It may be daunting at first, but how about telling us what type of programmes you have written before. What sort of coding are you comfortable with, are you familiar with security, networking etc.

I am sure if you can give us your main interests, you can get tons of suggestions as to which programmes to look at for your contributions. By searching gnu/linux bugs, there will be tons of overwhelming information because that term comprise many smaller programmes. Gnu/linux is not just one thing. We can later suggest a part of programmes that make up gnu/linux for you to look at.

Good luck.

As TobiSGD suggested, you first need to find a project to which you want to contribute. That project's homepage will give you links to the source code, mailing lists, bug tracking tool and other things you need in order to contribute.

A large number of open source projects are hosted at sourceforge. You can do various searches there to find a project you would like to work on.

Head to github, search for C/C++ related projects, fork them, look at the buglist, make fixes and submit a pull request. If they like what you've done, they will pull your changes into the branch or trunk.

Simple as that.