bpo-38425: Fix ‘res’ may be used uninitialized warning (pythonGH-16688)

(cherry picked from commit a05fcd3)

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>

Author: miss-islington

Parser/asdl_c.py

@@ -1209,7 +1209,6 @@ class PartingShots(StaticVisitor):
 /* mode is 0 for "exec", 1 for "eval" and 2 for "single" input */
 mod_ty PyAST_obj2mod(PyObject* ast, PyArena* arena, int mode)
 {
-    mod_ty res;
     PyObject *req_type[3];
     char *req_name[] = {"Module", "Expression", "Interactive"};
     int isinstance;
@@ -1231,6 +1230,8 @@ class PartingShots(StaticVisitor):
                      req_name[mode], Py_TYPE(ast)->tp_name);
         return NULL;
     }
+
+    mod_ty res = NULL;
     if (obj2ast_mod(ast, &res, arena) != 0)
         return NULL;
     else