Make sure we assume 1 core on faulty info

It appears that some OS's at the moment report 0 for nqp::cpucores.
Other places in the code already make sure there is at least 1 core.
It seemed appropriate to do the same check for the ThreadPoolScheduler.

src/core.c/ThreadPoolScheduler.pm6

@@ -589,7 +589,7 @@ my class ThreadPoolScheduler does Scheduler {
 
                 my @last-utils = 0e0 xx NUM_SAMPLES;
 #?endif
-                my int $cpu-cores = nqp::cpucores();
+                my int $cpu-cores = max(nqp::cpucores() - 1,1);
 
                 # These definitions used to live inside the supervisor loop.
                 # Moving them out of the loop does not improve CPU usage