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>

src/compiler/glsl/gl_nir_lower_samplers_as_deref.c

@@ -120,9 +120,10 @@ remove_struct_derefs_prep(nir_deref_instr **p, char **name,
 
 static void
 record_images_used(struct shader_info *info,
-                   nir_deref_instr *deref)
+                   nir_intrinsic_instr *instr)
 {
-   nir_variable *var = nir_deref_instr_get_variable(deref);
+   nir_variable *var =
+      nir_deref_instr_get_variable(nir_src_as_deref(instr->src[0]));
 
    /* Structs have been lowered already, so get_aoa_size is sufficient. */
    const unsigned size =
@@ -301,7 +302,7 @@ lower_intrinsic(nir_intrinsic_instr *instr,
       nir_deref_instr *deref =
          lower_deref(b, state, nir_src_as_deref(instr->src[0]));
 
-      record_images_used(&state->shader->info, deref);
+      record_images_used(&state->shader->info, instr);
 
       /* don't lower bindless: */
       if (!deref)