memory_limit is not always limited by max_memory_limit

Make sure to always duplicate max_memory_limit ini value. Otherwise the alter
ini routine may assume the value hasn't been overwritten, resulting in the
user-specified value being set after the on_modify handler has run.

Fixes phpGH-17951
Closes phpGH-19963

Author: manuelm

NEWS

@@ -6,6 +6,8 @@ PHP                                                                        NEWS
   . Sync all boost.context files with release 1.86.0. (mvorisek)
   . Fixed bug GH-20435 (SensitiveParameter doesn't work for named argument
     passing to variadic parameter). (ndossche)
+  . Fixed bug GH-17951 (memory_limit is not always limited by max_memory_limit).
+    (manuelm)
 
 - Standard:
   . Fix memory leak in array_diff() with custom type checks. (ndossche)

tests/basic/gh17951_runtime_change_6.phpt

@@ -0,0 +1,19 @@
+--TEST--
+GH-17951 Runtime Change 6
+--INI--
+memory_limit=128M
+max_memory_limit=512M
+--FILE--
+<?php
+for($i = 0; $i < 3; $i++) {
+  ini_set('memory_limit', '1024M');
+  echo ini_get('memory_limit');
+}
+?>
+--EXPECTF--
+Warning: Failed to set memory_limit to 1073741824 bytes. Setting to max_memory_limit instead (currently: 536870912 bytes) in %s on line %d
+512M
+Warning: Failed to set memory_limit to 1073741824 bytes. Setting to max_memory_limit instead (currently: 536870912 bytes) in %s on line %d
+512M
+Warning: Failed to set memory_limit to 1073741824 bytes. Setting to max_memory_limit instead (currently: 536870912 bytes) in %s on line %d
+512M