Properly escape error input in the message

Signed-off-by: Michal Čihař <michal@cihar.com>

libraries/plugins/transformations/abs/RegexValidationTransformationsPlugin.php

@@ -50,7 +50,7 @@ abstract class RegexValidationTransformationsPlugin extends IOTransformationsPlu
             $this->success = false;
             $this->error = sprintf(
                 __('Validation failed for the input string %s.'),
-                $buffer
+                htmlspecialchars($buffer)
             );
         }