tmux: use $terminfo to avoid echoti errors

See https://github.com/robbyrussell/oh-my-zsh/issues/7407#issuecomment-441665143

plugins/tmux/tmux.plugin.zsh

@@ -36,7 +36,7 @@ alias tkss='tmux kill-session -t'
 : ${ZSH_TMUX_FIXTERM_WITH_256COLOR:=screen-256color}
 
 # Determine if the terminal supports 256 colors
-if [[ $(echoti colors) == 256 ]]; then
+if [[ $terminfo[colors] == 256 ]]; then
   export ZSH_TMUX_TERM=$ZSH_TMUX_FIXTERM_WITH_256COLOR
 else
   export ZSH_TMUX_TERM=$ZSH_TMUX_FIXTERM_WITHOUT_256COLOR