gh-146102: Fix type slot_bf_getbuffer() error handling

Call PyBuffer_Release() if PyObject_GC_New() fails.

Fix also bytes_join(): only call Py_DECREF(item) after formatting the
error message which uses item.

Author: vstinner

Objects/stringlib/join.h

@@ -72,11 +72,11 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
                drops the sequence's last reference to it. */
             Py_INCREF(item);
             if (PyObject_GetBuffer(item, &buffers[i], PyBUF_SIMPLE) != 0) {
-                Py_DECREF(item);
                 PyErr_Format(PyExc_TypeError,
                              "sequence item %zd: expected a bytes-like object, "
                              "%.80s found",
                              i, Py_TYPE(item)->tp_name);
+                Py_DECREF(item);
                 goto error;
             }
             Py_DECREF(item);

Objects/typeobject.c

@@ -11327,6 +11327,7 @@ slot_bf_getbuffer(PyObject *self, Py_buffer *buffer, int flags)
 
     wrapper = PyObject_GC_New(PyBufferWrapper, &_PyBufferWrapper_Type);
     if (wrapper == NULL) {
+        PyBuffer_Release(buffer);
         goto fail;
     }
     wrapper->mv = ret;