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/helpers/plugin_functions.sh

@@ -31,9 +31,22 @@ _sourced_files() {
 		awk '/^[ \t]*source(-file)? +/ { gsub(/'\''/,""); gsub(/'\"'/,""); print $2 }'
 }
 
+# Want to be able to abort in certain cases
+trap "exit 1" TERM
+export TOP_PID=$$
+
+_fatal_error_abort() {
+	echo >&2 "Aborting."
+	kill -s TERM $TOP_PID
+}
+
 # PUBLIC FUNCTIONS BELOW
 
 tpm_path() {
+	if [ "$_CACHED_TPM_PATH" == "/" ]; then
+		echo >&2 "FATAL: Tmux Plugin Manager not configured in tmux.conf"
+		_fatal_error_abort
+	fi
 	echo "$_CACHED_TPM_PATH"
 }