Fixes server crash in chathistory limit parameter

[prdes]

When one provides a limit of -1 the prealloc gets this limit value, which is an invalid input to provide to make()

[slingamn]

Thanks for reporting this. This does not crash the server; it produces a panic on the client's own goroutine, which is handled here:

ergo/irc/client.go

Lines 662 to 670 in 5bab190

	if r := recover(); r != nil {
	client.server.logger.Error("internal",
	fmt.Sprintf("Client caused panic: %v\n%s", r, debug.Stack()))
	if client.server.Config().Debug.recoverFromErrors {
	client.server.logger.Error("internal", "Disconnecting client and attempting to recover")
	} else {
	panic(r)
	}
	}

so it causes the client to be disconnected. Since I think this can only be caused by a programming error, not triggered by another user's input, this is not a DoS. However, please be more careful in future: if you think you might have found a DoS, please contact me privately instead of opening a PR or issue.

[slingamn]

Closing this in favor of #2249