Panic on deleting first char in select mode

[antekresic]

There is a specific case when you delete the first character in select mode, library panics out:

`> panic: runtime error: integer divide by zero
[signal 0x8 code=0x7 addr=0x5958e pc=0x5958e]

goroutine 6 [running]:
panic(0x1306a0, 0xc82000a0b0)
/usr/local/Cellar/go/1.6.2/libexec/src/runtime/panic.go:481 +0x3e6
gopkg.in/readline%2ev1.(_opCompleter).doSelect(0xc820084080)
/go/src/gopkg.in/readline.v1/complete.go:49 +0xee
gopkg.in/readline%2ev1.(_opCompleter).OnComplete(0xc820084080)
/go/src/gopkg.in/readline.v1/complete.go:72 +0x1e6
gopkg.in/readline%2ev1.(*Operation).ioloop(0xc820060300)
/go/src/gopkg.in/readline.v1/operation.go:172 +0x1439
created by gopkg.in/readline%2ev1.NewOperation
/go/src/gopkg.in/readline.v1/operation.go:67 +0x42a`

Here is a gist of what I'm using to test this:
https://gist.github.com/antekresic/283106411e826f53799fa4565e90f094

Steps to reproduce: press 's', Tab, Tab, Backspace

Issue seems to be caused by entering OnComplete for an empty input rune buffer. My fix suggestion is to add a check for the empty buffer here:
https://github.com/chzyer/readline/blob/master/complete.go#L69

Let me know if you want a PR.

Thanks!

[chzyer]

Nice catch, Thanks! And PR is welcome :)