Fix crash when cfg.Painter is nil after another SetConfig() call

[tpodowd]

The following example code calls SetConfig() one more time after setting initializing readline instance with a different config. This is a contrived example, but this is useful when an application has a mode change and needs a different config (not just the prompt as in this example).

package main

import (
    "fmt"
    "strings"
    "github.com/chzyer/readline"
)

func main() {
    l, err := readline.NewEx(&readline.Config{})
    if err != nil {
        panic(err)
    }
    defer l.Close()

    l.SetConfig(&readline.Config{Prompt: "> "})
    line, err := l.Readline()
    fmt.Printf("you: %s\n", strings.TrimSpace(line))
}

Without the fix, this program will crash like this:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x4c23b4]

goroutine 1 [running]:
github.com/chzyer/readline.(*RuneBuffer).output(0xc0000b21e0, 0x51f648, 0xc0000b6050, 0x0)
    /home/tpodowd/work/readline/runebuf.go:494 +0xd4
github.com/chzyer/readline.(*RuneBuffer).print(0xc0000b21e0)
    /home/tpodowd/work/readline/runebuf.go:475 +0x2b
github.com/chzyer/readline.(*RuneBuffer).Refresh(0xc0000b21e0, 0x0)
    /home/tpodowd/work/readline/runebuf.go:465 +0xbf
github.com/chzyer/readline.(*Operation).Runes(0xc0000bc000, 0x0, 0x0, 0x0, 0x0, 0x0)
    /home/tpodowd/work/readline/operation.go:390 +0xf2
github.com/chzyer/readline.(*Operation).String(...)
    /home/tpodowd/work/readline/operation.go:377
github.com/chzyer/readline.(*Instance).Readline(0xc0000a0060, 0xc0000b4120, 0xc0000b4000, 0x0, 0x4649c5)
    /home/tpodowd/work/readline/readline.go:257 +0x2f
main.main()
    /home/tpodowd/work/readline/example/paint/paint.go:17 +0xdd
exit status 2