Bug fix to the zlib strategy tuning in multi-threaded mode. It could
(legitimately) drive the number of trials negative, but this meant it
never left the trial phase.


git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3531 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1