fix format truncation error with GCC-7 GCC-7 introduced new warnings and errors. Among them is a new warning for possible truncations in the output of snprintf. Since we are only interested in the return value of snprintf and do not use the string written by it we can also replace the buffer with a NULL pointer. This makes it explicit that we do not want to write a string and silences the GCC-7 error.
See also the examples in http://en.cppreference.com/w/c/io/fprintf
