do nothing if PENDING inputs are exist

Originally, zsh-syntax-highlighting called highlight functions at every inputs.
It's OK for user's keyboard inputs. But when long inputs come from clipboard,
it cause noticeable slowdown.

zsh-syntax-highlighting.zsh

@@ -52,6 +52,10 @@ typeset -a zsh_highlight_predicates; zsh_highlight_predicates=()
 typeset -a zsh_highlight_caches; zsh_highlight_caches=()
 
 _zsh_highlight-zle-buffer() {
+  if (( PENDING )); then
+    return
+  fi
+
   local ret=$?
   {
     local -a funinds