BUILD: task: silence a build warning with threads disabled

BUILD: task: silence a build warning with threads disabled

The compiler doesn't know that a random value based on global.nbthread
is necessarily smaller than MAX_THREADS, and when picking a random
thread number while single-threaded it complains that new_tid 1 is
out of bounds for the array. In fact all this is dead code in this
case.

Let's tell it about it to silence the warning.