libfru: char Str::tokenize should return NUL char, not NULL pointer
This is small fallout from 8017 Comply with POSIX.1-2008 and C++11 for the definition of NULL, building with gcc 6 reveals:
../libgenutil/Str.cc: In member function 'char Str::tokenize(Str&, const Str&, Str&)':
../libgenutil/Str.cc:174:14: error: cannot convert 'std::nullptr_t' to 'char' in return
Which of course is bug in the code, we should return '\0' here.