Minor updates to @michael-ekstrand answer

1. Prevent unnecessary function definitions
2. Wrap expensive tmux calls with simnple builtin tests
3. Prevent error if $SSH_AUTH_SOCK contains whitespace

if [[ -n $TMUX ]]; then
  _update_ssh_agent() {
    if ! [[ -S $SSH_AUTH_SOCK ]]; then
      eval "$(tmux show-environment -s SSH_AUTH_SOCK 2>/dev/null)"
    fi
  }
  add-zsh-hook precmd _update_ssh_agent
fi