Fix Shellcheck SC2115: Ensure never expands to /*

Use "${var:?}" to ensure this never expands to /* .

https://github.com/koalaman/shellcheck/wiki/SC2115

Signed-off-by: Dan Callahan <danc@element.io>

demo/clean.sh

@@ -12,8 +12,8 @@ if [ -f $PID_FILE ]; then
 fi
 
 for port in 8080 8081 8082; do
-    rm -rf $DIR/$port
+    rm -rf ${DIR:?}/$port
     rm -rf $DIR/media_store.$port
 done
 
-rm -rf $DIR/etc
+rm -rf ${DIR:?}/etc