Fix bug in previous commit (d330b49b33)

'local' is a reserved word in zsh 5.1 but not in earlier versions [1].
Therefore, under zsh older than 5.1, quoting is required.

This manifested as random «builtin=''» in emitted to the terminal, and
commands (such as 'echo') highlighted as errors (in red).

[1] https://github.com/zsh-users/zsh/blob/master/README#L46
    (the section "Incompatibilites between 5.0.8 and 5.1")

highlighters/main/main-highlighter.zsh

@@ -147,7 +147,7 @@ _zsh_highlight_main_highlighter()
       style=$ZSH_HIGHLIGHT_STYLES[precommand]
       sudo=true
      else
-      local res=$(LC_ALL=C builtin type -w $arg 2>/dev/null)
+      local res="$(LC_ALL=C builtin type -w $arg 2>/dev/null)"
       case $res in
         *': reserved')  style=$ZSH_HIGHLIGHT_STYLES[reserved-word];;
         *': alias')     style=$ZSH_HIGHLIGHT_STYLES[alias]