fix(_get_comp_words_by_ref): work around localvar_inherit

[akinomyoga]

I separate the change from #790 #791. This is the most independent part.

• 868e98e work around localvar_inherit: This is actually not directly related to the refactoring of _command_offset, but I noticed this in investigating which variables can be modified by _comp_offset for 868e98e. The implementation of _get_comp_words_by_ref assumes that the declared variables vcur, vwords, vcword, and vprev have initially unset values to later test them with [[ -v vcur ]], etc., but this is not ensured when localvar_inherit is turned on.

The description is unchanged from that in #790 #791.