DERBY-6714: RuntimeInfoTest failed with insufficient data from server

Tighten up checks for sessions being closed while reading runtime info from
them, in order to protect against NullPointerExceptions on the server.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1620608 13f79535-47bb-0310-9956-ffa450edef68