posix.lua: fix a typo in assert.
posix.lua: fix a typo in assert.

* posix.lua (pipeline): t1 is not declared, use type of t in the
assert error.

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