It's not needed in XZ Utils at least for now. It's good to support
it still because if such use is needed later, it wouldn't be
caught on GNU/Linux since malloc(0) from glibc returns non-NULL.
Warnings about unused tuktest_run_test conveniently tell which
test programs haven't been converted to tuktest.h yet but I
silenced that warning too for now anyway.
It is fine to use __attribute__((__unused__)) even when the
function is actually used because the attribute only means
that the function might be unused.
