Simplify getbuffer(): convertbuffer() fails anyway if bf_getbuffer is NULL

Python/getargs.c

@@ -1410,7 +1410,7 @@ convertsimple(PyObject *arg, const char **p_format, va_list *p_va, int flags,
 static Py_ssize_t
 convertbuffer(PyObject *arg, void **p, char **errmsg)
 {
-    PyBufferProcs *pb = arg->ob_type->tp_as_buffer;
+    PyBufferProcs *pb = Py_TYPE(arg)->tp_as_buffer;
     Py_ssize_t count;
     Py_buffer view;
 
@@ -1438,14 +1438,15 @@ convertbuffer(PyObject *arg, void **p, char **errmsg)
 static int
 getbuffer(PyObject *arg, Py_buffer *view, char **errmsg)
 {
-    void *buf;
-    Py_ssize_t count;
-    PyBufferProcs *pb = arg->ob_type->tp_as_buffer;
+    PyBufferProcs *pb = Py_TYPE(arg)->tp_as_buffer;
     if (pb == NULL) {
         *errmsg = "bytes or buffer";
         return -1;
     }
-    if (pb->bf_getbuffer) {
+    if (pb->bf_getbuffer == NULL) {
+        *errmsg = "convertible to a buffer";
+        return -1;
+    }
     if (PyObject_GetBuffer(arg, view, 0) < 0) {
         *errmsg = "convertible to a buffer";
         return -1;
@@ -1455,15 +1456,6 @@ getbuffer(PyObject *arg, Py_buffer *view, char **errmsg)
         return -1;
     }
     return 0;
-    }
-
-    count = convertbuffer(arg, &buf, errmsg);
-    if (count < 0) {
-        *errmsg = "convertible to a buffer";
-        return count;
-    }
-    PyBuffer_FillInfo(view, NULL, buf, count, 1, 0);
-    return 0;
 }
 
 /* Support for keyword arguments donated by