dollar-double-quoted-argument: Handle «"foo$"» correctly.

highlighters/main/main-highlighter.zsh

@@ -272,6 +272,8 @@ _zsh_highlight_main_highlighter_highlight_string()
             if [[ ${arg:$i} =~ ^([A-Za-z_][A-Za-z0-9_]*|[0-9]+) ]] ; then
               (( k += $#MATCH )) # highlight the parameter name
               (( i += $#MATCH )) # skip past it
+            else
+              continue
             fi
             ;;
       "\\") style=$ZSH_HIGHLIGHT_STYLES[back-double-quoted-argument]