Fix string_size

Revert the string_size change. Instead, restore string_size to its
previous value on error.

Author: vstinner

Modules/_io/bytesio.c

@@ -759,10 +759,12 @@ _io_BytesIO_truncate_impl(bytesio *self, PyObject *size)
     }
 
     if (new_size < self->string_size) {
+        Py_ssize_t old_string_size = self->string_size;
+        self->string_size = new_size;
         if (resize_buffer_lock_held(self, new_size) < 0) {
+            self->string_size = old_string_size;
             return NULL;
         }
-        self->string_size = new_size;
     }
 
     return PyLong_FromSsize_t(new_size);