Prevent clean_plugins from deleting root

If tmux was not yet configured to initialize the Tmux Plugin Manager,
the variable for the plugin home was not being set. This was resulting
in a `rm -rf /` which is pretty nuclear.

Fixes: https://github.com/tmux-plugins/tpm/issues/58

scripts/update_plugin.sh

@@ -60,6 +60,7 @@ update_plugins() {
 }
 
 main() {
+	ensure_tpm_path_exists
 	if [ "$1" == "all" ]; then
 		update_all
 	else