Fix fetching default value of internal function with userland arginfo

"Fix" in the sense of "not crash". We aren't able to actually
display the default value for this case, as there's no way to
fetch the relevant information right now.

ext/reflection/php_reflection.c

@@ -667,7 +667,10 @@ static void _parameter_string(smart_str *str, zend_function *fptr, struct _zend_
 	if (!required) {
 		if (fptr->type == ZEND_INTERNAL_FUNCTION) {
 			smart_str_appends(str, " = ");
-			if (((zend_internal_arg_info*)arg_info)->default_value) {
+			/* TODO: We don't have a way to fetch the default value for an internal function
+			 * with userland arg info. */
+			if (has_internal_arg_info(fptr)
+					&& ((zend_internal_arg_info*)arg_info)->default_value) {
 				smart_str_appends(str, ((zend_internal_arg_info*)arg_info)->default_value);
 			} else {
 				smart_str_appends(str, "<default>");

ext/reflection/tests/default_value_internal_userland_arginfo.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Fetching default value of an internal trampoline function with userland arginfo
+--FILE--
+<?php
+$closure = function ($b = 0) {};
+$ro = new ReflectionObject($closure);
+$rm = $ro->getMethod('__invoke');
+echo $rm, "\n";
+?>
+--EXPECT--
+Method [ <internal> public method __invoke ] {
+
+  - Parameters [1] {
+    Parameter #0 [ <optional> $b = <default> ]
+  }
+}