Skip counting currentNesting if null

This is an error starting in PHP 7.2

library/HTMLPurifier/Injector.php

@@ -157,6 +157,7 @@ abstract class HTMLPurifier_Injector
             return false;
         }
         // check for exclusion
+        if (!empty($this->currentNesting)) {
             for ($i = count($this->currentNesting) - 2; $i >= 0; $i--) {
                 $node = $this->currentNesting[$i];
                 $def  = $this->htmlDefinition->info[$node->name];
@@ -164,6 +165,7 @@ abstract class HTMLPurifier_Injector
                     return false;
                 }
             }
+        }
         return true;
     }