Add regression tests for bug #79836

Co-authored-by: changochen1@gmail.com

Author: nielsdos

Zend/tests/bug79836.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Bug #79836 (Segfault in concat_function)
+--FILE--
+<?php
+error_reporting(E_ALL ^ E_WARNING);
+ob_start ( function () use ( & $c ) { $c = 0 ;}, 1 );
+$c .= [];
+$c .= [];
+ob_end_clean();
+echo "Done\n";
+?>
+--EXPECT--
+Done

Zend/tests/bug79836_1.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Bug #79836 (Segfault in concat_function)
+--FILE--
+<?php
+error_reporting(E_ALL ^ E_WARNING);
+$x = 'non-empty';
+ob_start ( function () use ( & $x ) { $x = 0 ;}, 1 );
+$c = [];
+$x = $c . $x;
+$x = $c . $x;
+ob_end_clean();
+echo "Done\n";
+?>
+--EXPECT--
+Done