compiler/glsl: avoid null-pointer deref
When we encounter a bindless image here, lower_deref returns a NULL-pointer, and calling record_images_used will try to dereference that NULL-pointer. So let's dig out the var from the source instruction instead of the result of the lowering. Fixes: 5910c938 ("nir/glsl: gather bitmask of images used by program") Reviewed-by:Tapani Pälli <tapani.palli@intel.com> Part-of: <mesa/mesa!9895> (cherry picked from commit 89a04a54)