Security: Invalid reads, always add NULL for compatibility.

Security: Invalid reads, always add NULL for compatibility.

After switching from f_string_t to f_string_static_t, the NULL terminated string problem has become apparent.
If at any point in time these strings are passed to a standard function that expects NULL terminated strings, then an invalid read (or write) can occur.

Manually appending a NULL termination every time its needed has made the code messier than I would like.
This commit changes the behavior to instead always append a NULL termination after the string.used when appending strings.
I accept the additional resource cost of 1-byte per string to guarantee this.

This should make the program more easily more secure by catering to the NULL terminated string code out there.
This project still doesn't need the or care about NULL termination for most (but not all) of its functions.