glsl: Don't pass NULL to ir_assignment constructor when not necessary

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>

src/compiler/glsl/lower_texture_projection.cpp

@@ -69,7 +69,7 @@ lower_texture_projection_visitor::visit_leave(ir_texture *ir)
 						    ir->projector->type,
 						    ir->projector,
 						    NULL);
-   ir_assignment *assign = new(mem_ctx) ir_assignment(deref, expr, NULL);
+   ir_assignment *assign = new(mem_ctx) ir_assignment(deref, expr);
    base_ir->insert_before(assign);
 
    deref = new(mem_ctx) ir_dereference_variable(var);