tmux: fix invalid syntax on old zsh versions

First reported on https://github.com/robbyrussell/oh-my-zsh/commit/f584de5930467fd53e8b7d2e51f5227bc405e4b2#r29984052

plugins/tmux/tmux.plugin.zsh

@@ -56,7 +56,8 @@ function _zsh_tmux_plugin_run() {
     return $?
   fi
 
-  local -a tmux_cmd=(command tmux)
+  local -a tmux_cmd
+  tmux_cmd=(command tmux)
   [[ "$ZSH_TMUX_ITERM2" == "true" ]] && tmux_cmd+=(-CC)
 
   # Try to connect to an existing session.