[3.13] gh-137571: Protect against possible UnboundLocalError in gzip._GzipReader.read() (GH-150222) (GH-150231)

This has not been observed in practice, but we cannot be 100% sure that
it will not happen with some weird gzip data.
(cherry picked from commit 28eac9a7263ad8dcfa9b536aa238549131857e0f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>