tu: Fix xfb when there is a hole at the end

We were handling the case where we had an unassigned output in the middle of the outputs array, but v->outputs can be smaller than the shader's info.num_outputs when an output at the end isn't assigned. This lead to us reading garbage after the end, and assuming that it corresponded to r0.x and overwriting the xfb entry for some other random output with the unassigned output's entry.