bpo-43321: Fix SystemError in getargs.c (GH-24656)

Misc/NEWS.d/next/Core and Builtins/2021-02-26-13-17-52.bpo-43321.TCS3ph.rst

@@ -0,0 +1,2 @@
+Fix ``SystemError`` raised when ``PyArg_Parse*()`` is used with ``#`` but
+without ``PY_SSIZE_T_CLEAN`` defined.

Python/getargs.c

@@ -655,14 +655,14 @@ static const char *
 convertsimple(PyObject *arg, const char **p_format, va_list *p_va, int flags,
               char *msgbuf, size_t bufsize, freelist_t *freelist)
 {
+#define RETURN_ERR_OCCURRED return msgbuf
     /* For # codes */
 #define REQUIRE_PY_SSIZE_T_CLEAN \
     if (!(flags & FLAG_SIZE_T)) { \
         PyErr_SetString(PyExc_SystemError, \
                         "PY_SSIZE_T_CLEAN macro must be defined for '#' formats"); \
-        return NULL; \
+        RETURN_ERR_OCCURRED; \
     }
-#define RETURN_ERR_OCCURRED return msgbuf
 
     const char *format = *p_format;
     char c = *format++;