Fix null-pointer deref in if stmt printing

Fixes OSS-Fuzz #17721.

Author: nikic

Zend/tests/assert/expect_015.phpt

@@ -140,6 +140,7 @@ assert(0 && ($a = function () {
     if ($a) {
     } elseif ($b) {
     }
+    if ($a); else;
 }));
 
 ?>
@@ -292,4 +293,7 @@ Warning: assert(): assert(0 && ($a = function () {
     if ($a) {
     } elseif ($b) {
     }
+    if ($a) {
+    } else {
+    }
 })) failed in %sexpect_015.php on line %d

Zend/zend_ast.c

@@ -894,7 +894,7 @@ static void zend_ast_export_if_stmt(smart_str *str, zend_ast_list *list, int ind
 		} else {
 			zend_ast_export_indent(str, indent);
 			smart_str_appends(str, "} else ");
-			if (ast->child[1]->kind == ZEND_AST_IF) {
+			if (ast->child[1] && ast->child[1]->kind == ZEND_AST_IF) {
 				list = (zend_ast_list*)ast->child[1];
 				goto tail_call;
 			} else {