Check if each variable is in the symbol table.

Delete if not before passing into compact.

lib/Minify/App.php

@@ -80,6 +80,11 @@ class App extends Container
             };
             $varNames = array_map($prefixer, $propNames);
 
+            $varDefinedChecker = function ($name) {
+                return array_key_exists($name, get_defined_vars());
+            };
+            $varNames = array_filter($varNames, $varDefinedChecker);
+
             $vars = compact($varNames);
 
             foreach ($varNames as $varName) {