fixes #1545 avoid fatal error during upgrade from 11 to 12

include/smarty/libs/sysplugins/smarty_internal_undefined.php

@@ -61,6 +61,15 @@ class Smarty_Internal_Undefined
         if (isset($this->class)) {
             throw new SmartyException("undefined extension class '{$this->class}'");
         } else {
+            // Piwigo specifics - starts here
+            // when updating from Piwigo 11 to 12, we try to delete compiled templates and there is a mix old and new Smarty files, resulting in a:
+            // Fatal error: Uncaught --> Smarty: Smarty->_clearTemplateCache() undefined method
+            // indeed this method does not exist in Smarty 3.1.29 (Piwigo 11) but exists in Smarty 3.1.39 (Piwigo 12)
+            if ('_clearTemplateCache' == $name)
+            {
+                return;
+            }
+            // Piwigo specifics - stops here
             throw new SmartyException(get_class($args[ 0 ]) . "->{$name}() undefined method");
         }
     }