fix STARPU_LIMIT_CPU_MEM behavior when hwloc is not enabled

src/drivers/cpu/driver_cpu.c

@@ -170,7 +170,7 @@ static size_t _starpu_cpu_get_global_mem_size(int nodeid STARPU_ATTRIBUTE_UNUSED
 	if (limit < 0)
 		// No limit is defined, we return the global memory size
 		return global_mem;
-	else if ((size_t)limit * 1024*1024 > global_mem)
+	else if (global_mem && (size_t)limit * 1024*1024 > global_mem)
 		// The requested limit is higher than what is available, we return the global memory size
 		return global_mem;
 	else