mem: add 0 check to darwin.swapmem.

mem/mem_darwin.go

@@ -92,7 +92,10 @@ func SwapMemory() (*SwapMemoryStat, error) {
 		return nil, err
 	}
 
-	u := ((total_v - free_v) / total_v) * 100.0
+	u := float64(0)
+	if total_v != 0 {
+		u = ((total_v - free_v) / total_v) * 100.0
+	}
 
 	// vm.swapusage shows "M", multiply 1000
 	ret = &SwapMemoryStat{