Quote variable to highlight right words on zsh 4.3

When the variable is not quoted, there seems to be some difference in
evaluating the line between zsh 4.3 and 5.6

Author: d10n

zsh-history-substring-search.zsh

@@ -324,7 +324,7 @@ _history-substring-search-end() {
     for query_part in $_history_substring_search_query_parts; do
       local escaped_query_part=${query_part//(#m)[\][()|\\*?#<>~^]/\\$MATCH}
       # (i) get index of pattern
-      local query_part_match_index=${${BUFFER:$highlight_start_index}[(i)(#$HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS)${escaped_query_part}]}
+      local query_part_match_index="${${BUFFER:$highlight_start_index}[(i)(#$HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS)${escaped_query_part}]}"
       if [[ $query_part_match_index -le ${#BUFFER:$highlight_start_index} ]]; then
         highlight_start_index=$(( $highlight_start_index + $query_part_match_index ))
         highlight_end_index=$(( $highlight_start_index + ${#query_part} ))