nir/lower_blend: Don't dereference null

If a dual source blend colour is never written, src1 will be null and it will be invalid to dereference it. src1 is dereferenced both for the f2fN instruction but also if a dual blend factor is used... even if the latter isn't strictly valid, segfaulting in the NIR pass seems a lot meaner than blending with an undef.

The referenced commit hosed Asahi, causing anything that used blending to crash. Panfrost is unaffected since it always supplies a dual colour due to our crude construction of blend shaders.

Fixes: 83130165 ("nir/lower_blend: Consume dual stores") Signed-off-by: Alyssa Rosenzweig alyssa@rosenzweig.io