posix.lua: fix bug in posix.pipeline_iterator, where it always closed the pipe on first iteration
posix.lua: fix bug in posix.pipeline_iterator, where it always closed the pipe on first iteration

luaposix/luaposix,jimmyharris/luaposix,zevv/luaposix,rrthomas/luaposix,ardje/luaposix,rrthomas/luaposix,snegovick/luaposix,zevv/luaposix,snegovick/luaposix,jimmyharris/luaposix,ardje/luaposix