Fix variable leakage into global namespace.
Fix variable leakage into global namespace.

Similar to zsh-users/zsh-syntax-highlighting#97.

zsh-users/zsh-syntax-highlighting,fly19890211/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,stuha/wbr-zsh,joshuarubin/dotfiles,joshuarubin/dotfiles,m0vie/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,m0vie/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting-debian,P71/zsh-syntax-highlighting,joshuarubin/dotfiles,x-way/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting-debian,ryexley/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,g5pw/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting,joshuarubin/dotfiles