fix some bugs

According to the question of exercise 4.15, the point pi should be 0.   
In the exercise 4.30, the parenthesize `sizeof f()` can be compiled successfully. The sizeof operator returns  the size, in bytes, of an expression or a type name.   
PS. I forget the return of f(). If the return of f() is void, compile error. OMG~~~
So, how about this answer? @Mooophy We should changed Mooophy/Cpp-Primer#35 's answer?