Xiao Wei

Fix: panic on accessing nil error

1

## panic 1 ``` goroutine 83 [running]: github.com/henson/proxypool/getter.IP89(0x0, 0x0, 0x0) /Users/vicxiao/workspace/go/github/proxypool/getter/89ip.go:27 +0x453 main.run.func1(0xc0002ba1e0, 0xc0002a6480, 0x4768f98) ``` sample output after fix ``` 2020/05/07 11:36:45 [ WARN] failed to get proxies from...