Fix memory leaks in zlib.compress() and .decompress().

Also, make sure that test_zlib tests decompress() for overly-large inputs.
diff --git a/Lib/test/test_zlib.py b/Lib/test/test_zlib.py
index 852857b..48d9f58 100644
--- a/Lib/test/test_zlib.py
+++ b/Lib/test/test_zlib.py
@@ -193,6 +193,7 @@
         data = b'x' * size
         try:
             self.assertRaises(OverflowError, zlib.compress, data, 1)
+            self.assertRaises(OverflowError, zlib.decompress, data)
         finally:
             data = None