Server doesn't handle negative offset and/or limit given to Connz

[kozlovic]

If server is started with monitoring on, and an user queries the connz endpoint with negative offset and/or limit, the server will report this for the current go-routine handling the request. The server is still working properly (does not entirely panic) and other well formed requests will go through.

For instance: http://localhost:8222/connz?offset=-1 produces this:

2017/05/17 11:19:42 http: panic serving [::1]:60778: runtime error: slice bounds out of range
goroutine 34 [running]:
net/http.(*conn).serve.func1(0xc42016c000)
	/usr/local/go/src/net/http/server.go:1491 +0x12a
panic(0x350c80, 0xc420012100)
	/usr/local/go/src/runtime/panic.go:458 +0x243
github.com/nats-io/gnatsd/server.(*Server).HandleConnz(0xc420136280, 0x544820, 0xc4201a40d0, 0xc42019a000)
	/Users/ivan/dev/go/src/github.com/nats-io/gnatsd/server/monitor.go:150 +0x19fb
github.com/nats-io/gnatsd/server.(*Server).HandleConnz-fm(0x544820, 0xc4201a40d0, 0xc42019a000)
	/Users/ivan/dev/go/src/github.com/nats-io/gnatsd/server/server.go:492 +0x48
net/http.HandlerFunc.ServeHTTP(0xc420013550, 0x544820, 0xc4201a40d0, 0xc42019a000)
	/usr/local/go/src/net/http/server.go:1726 +0x44
net/http.(*ServeMux).ServeHTTP(0xc42000d650, 0x544820, 0xc4201a40d0, 0xc42019a000)
	/usr/local/go/src/net/http/server.go:2022 +0x7f
net/http.serverHandler.ServeHTTP(0xc420096380, 0x544820, 0xc4201a40d0, 0xc42019a000)
	/usr/local/go/src/net/http/server.go:2202 +0x7d
net/http.(*conn).serve(0xc42016c000, 0x544fa0, 0xc42017e080)
	/usr/local/go/src/net/http/server.go:1579 +0x4b7
created by net/http.(*Server).Serve
	/usr/local/go/src/net/http/server.go:2293 +0x44d