AddIP/RemoveIP() AddIPAddr/RemoveIPAddr() could make a panic in concurrent environment

[snaix]

Hi
I found that fastping could panic when call RemoveIP() in other goroutine:
fastping fatal error: concurrent map iteration and map write
The panic point in sendICMP():

	...
	for key, addr := range p.addrs {
	...

Which an unprotected p.addrs in concurrent enviroment.
So when I was running p.RunLoop() without p.Stop(), and call p.RemoveIP() in another goroutine.
A panic will happen:
fastping fatal error: concurrent map iteration and map write