bpo-31848: Fix broken error handling in Aifc_read.initfp() when the SSND chunk is not found (#5240)

Initialize self._ssnd_chunk so that aifc.Error is raised as intended,
not AttributeError.
diff --git a/Misc/ACKS b/Misc/ACKS
index ea1d941..b15b20e 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1512,6 +1512,7 @@
 Per Spilling
 Joshua Spoerri
 Noah Spurrier
+Zackery Spytz
 Nathan Srebro
 RajGopal Srinivasan
 Tage Stabell-Kulo