THRIFT-929. cpp: Fix a couple minor issues in ZlibTest

- Add a necessary cast.
- Check buffer sizes more carefully to allow for different read/write
  size distributions.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@1005147 13f79535-47bb-0310-9956-ffa450edef68
1 file changed