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.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12310>
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