Fix #2256 Invalid utf8 causing partially missing output

Added ENT_SUBSTITUTE parameter to htmlspecialchars in Printer->write

Author: radiat-r

src/Util/Printer.php

@@ -128,7 +128,7 @@ public function write($buffer)
             }
         } else {
             if (PHP_SAPI != 'cli' && PHP_SAPI != 'phpdbg') {
-                $buffer = htmlspecialchars($buffer);
+                $buffer = htmlspecialchars($buffer, ENT_SUBSTITUTE);
             }
 
             print $buffer;