Repeated PushFront -> PopBack causes index out of range [-1]

[ghost]

This panics on the 65th iteration:

package main

import "github.com/ef-ds/deque"

func main() {
	q := deque.New()
	for {
		q.PushFront(1)
		q.PopBack()
	}
}

https://play.golang.org/p/CaG4fu1EY-E

I know that isn't the normal use, but I feel like it should work.

[christianrpetrin]

Many thanks @jrodatus for reporting the issue!