Fix GH-16414: zend_test.observer.observe_function_names may segfault

Unless `zend_test.observer.enabled` is on, we must not add observer
handlers, so we let the INI modify handler fail early.

Proper fix suggested by @bwoebi.

Author: cmb69

ext/zend_test/observer.c

@@ -304,6 +304,9 @@ static ZEND_INI_MH(zend_test_observer_OnUpdateCommaList)
 	zend_array **p = (zend_array **) ZEND_INI_GET_ADDR();
 	zend_string *funcname;
 	zend_function *func;
+	if (!ZT_G(observer_enabled)) {
+		return FAILURE;
+	}
 	if (stage != PHP_INI_STAGE_STARTUP && stage != PHP_INI_STAGE_ACTIVATE && stage != PHP_INI_STAGE_DEACTIVATE && stage != PHP_INI_STAGE_SHUTDOWN) {
 		ZEND_HASH_FOREACH_STR_KEY(*p, funcname) {
 			if ((func = zend_hash_find_ptr(EG(function_table), funcname))) {

ext/zend_test/tests/gh16414.phpt

@@ -0,0 +1,11 @@
+--TEST--
+GH-16414 (zend_test.observer.observe_function_names may segfault)
+--EXTENSIONS--
+zend_test
+--FILE--
+<?php
+function bar() {}
+var_dump(ini_set("zend_test.observer.observe_function_names", "bar"));
+?>
+--EXPECT--
+bool(false)