cope with more accelerators than cpus

src/core/topology.c

@@ -446,6 +446,8 @@ static int _starpu_init_machine_config(struct starpu_machine_config_s *config,
 	if (explicitval < 0) {
 		unsigned already_busy_cpus = (topology->ngordon_spus?1:0) + topology->ncudagpus + topology->nopenclgpus;
 		long avail_cpus = topology->nhwcpus - (use_accelerator?already_busy_cpus:0);
+		if (avail_cpus < 0)
+			avail_cpus = 0;
 		topology->ncpus = STARPU_MIN(avail_cpus, STARPU_NMAXCPUS);
 	} else {
 		/* use the specified value */