[ticket/10188] Use ob_get_level in msg_handler for output buffering check.

Output buffering may be enabled via various approaches, among them:

 * output_buffering in php.ini;
 * output_handler in php.ini enables output_buffering;
 * ob_start call.

ob_get_level allows us to query php runtime for the actual output buffering
status.

PHPBB3-10188

phpBB/includes/functions.php

@@ -3743,7 +3743,7 @@ function msg_handler($errno, $msg_text, $errfile, $errline)
 			if (strpos($errfile, 'cache') === false && strpos($errfile, 'template.') === false)
 			{
 				// flush the content, else we get a white page if output buffering is on
-				if ((int) @ini_get('output_buffering') === 1 || strtolower(@ini_get('output_buffering')) === 'on')
+				if (ob_get_level() > 0)
 				{
 					@ob_flush();
 				}