main: Return 0 if braces_stack is empty
main: Return 0 if braces_stack is empty

This allows a caller to know if the command or process substitution is
complete.

joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,joshuarubin/dotfiles,P71/zsh-syntax-highlighting,stuha/wbr-zsh,x-way/zsh-syntax-highlighting,joshuarubin/dotfiles,joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting