Fix SF #544995 (zlib crash on second flush call)
Bug fix by mhammond.

Bug fix candidate for 2.2, not present in 2.1.
diff --git a/Lib/test/test_zlib.py b/Lib/test/test_zlib.py
index 8881516..35bca88 100644
--- a/Lib/test/test_zlib.py
+++ b/Lib/test/test_zlib.py
@@ -41,6 +41,12 @@
 co = zlib.compressobj(8, 8, -15)
 x1 = co.compress(buf)
 x2 = co.flush()
+try:
+    co.flush()
+    print "Oops - second flush worked when it should not have!"
+except zlib.error:
+    pass
+
 x = x1 + x2
 
 dc = zlib.decompressobj(-15)