`snprintf` returns the number of characters that would have been written
had the buffer been large enough.
It's a common trick to call `snprintf(nullptr, 0, ...)` to measure how
large a buffer has to be.
Thus the return value is not zero but fourteen.
This enables a nice warning in case a function becomes dead code. Also,
in the case of test-crypto.cpp, I took the liberty to add the prefix 'g_'
to the global event loop.
