driver: Tolerate KSH_ARRAYS being set in the calling context.
driver: Tolerate KSH_ARRAYS being set in the calling context.

Fixes zsh-users/zsh-syntax-highlighting#162.

phy1729/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,m0vie/zsh-syntax-highlighting,joshuarubin/dotfiles,x-way/zsh-syntax-highlighting,joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting,stuha/wbr-zsh,m0vie/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,joshuarubin/dotfiles,danielshahaf/zsh-syntax-highlighting-debian,danielshahaf/zsh-syntax-highlighting-debian,psprint/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,joshuarubin/dotfiles,x-way/zsh-syntax-highlighting