driver: Fix a bug that prevented subsequent, third-party zle-line-pre-redraw hooks from running.
driver: Fix a bug that prevented subsequent, third-party zle-line-pre-redraw hooks from running.

Without this patch, `_zsh_highlight` was invoked by add-zle-hook-widget
with `$?` being non-zero (see add-zle-hook-widget:48-52).  Since
`_zsh_highlight` preserves `$?` from its caller's point of view,
add-zle-hook-widget saw a non-zero exit code from `_zsh_highlight` and
did not run any the remaining zle-line-pre-redraw hooks.

See https://github.com/zsh-users/zsh-syntax-highlighting/issues/579#issuecomment-623576907.

P71/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting