Passing a null pointer to the "%s" argument of a printf-like
function is undefined behaviour. In the GNU libc implementation
of the printf() family of functions, it happens to work.

GCC 10.2.0 would diagnose this with -Wformat-overflow -Og.
In -fsanitize=undefined (WITH_UBSAN=ON) builds, a runtime error
would be generated. In some other builds, GCC 8 or later might infer
that the parameter is nonnull and optimize away further checks whether
the parameter is null, leading to SIGSEGV.