Fixed bug #70876 Segmentation fault when regenerating session id with strict mode

The comment *mod_data always be non-NULL is not true.
The same as this FIXME: https://github.com/php/php-src/blob/master/ext/session/mod_files.c#L676

ext/session/mod_files.c

@@ -702,6 +702,10 @@ PS_VALIDATE_SID_FUNC(files)
 {
 	PS_FILES_DATA;
 
+	if (!data) {
+		return FAILURE;
+	}
+
 	return ps_files_key_exists(data, ZSTR_VAL(key));
 }
 

ext/session/tests/bug70876.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Bug #70876 Segmentation fault when regenerating session id with strict mode
+--SKIPIF--
+<?php include('skipif.inc'); ?>
+--INI--
+html_errors=0
+session.save_handler=files
+--FILE--
+<?php
+ini_set('session.use_strict_mode', true);
+session_start();
+session_regenerate_id();
+
+echo "ok";
+?>
+--EXPECT--
+ok