[2.7] bpo-31848: Fix broken error handling in Aifc_read.initfp() when the SSND chunk is not found (GH-5240) (GH-5781)

Initialize self._ssnd_chunk so that aifc.Error is raised as intended,
not AttributeError.
(cherry picked from commit 80d20b91)
Co-authored-by: Zackery Spytz <zspytz@gmail.com>