There is no need to learn C before learning C++.
They are different languages. It is a common misconception that C++ is in some way dependent on C and not a fully specified language on its own.
Just because C++ shares a lot of the same syntax and a lot of the same semantics, does not mean you need to learn C first.
If you learn C++ you will eventually learn most of C with some differences between the languages that you will learn over time. In fact its a very hard thing to write proper C++ because intermediate C++ programmers tend to write C/C++.That is true whether or not you started with C or started with C++.
If you know C first, then that is good plus to learning C++. You will start with knowing a chunk of the language. If you do not know C first then there is no point focusing on a different language. There are plenty of good books and tutorials available that start you from knowing nothing and will cover anything you would learn from C which applies to C++ as well.
Please see further reasoning in this answer.