Do not update pos if peek_bytes failed

Author: marcelm

Modules/_io/bytesio.c

@@ -451,7 +451,9 @@ static PyObject *
 read_bytes_lock_held(bytesio *self, Py_ssize_t size)
 {
     PyObject *bytes = peek_bytes_lock_held(self, size);
-    self->pos += size;
+    if (bytes != NULL) {
+        self->pos += size;
+    }
     return bytes;
 }