Use nullptr instead of zero

[rilysh]

Hello,
Instead of using integer 0 (zero) for char *, FILE *, and function argument (setvbuf), mentioning nullptr seems reasonable for C++. Since in C, NULL is defined as (void *)0, which I assume should be similar to C++ too, this explicitly removes the warning which can be viewed by the -Wzero-as-null-pointer-constant parameter for both, GCC and Clang.