Use "int" rather than "unsigned" as the argument to ssh2_set_window, not

because it can ever be negative, but because we'll be comparing it with
another int.  This way, C's promotion rules don't bite us and we should
stand slightly more chance of coping with broken servers that overrun our
window.

git-svn-id: svn://svn.tartarus.org/sgt/putty@7683 cda61777-01e9-0310-a592-d414129be87e