glsl: silence uninitialized var warning on MinGW

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/glsl/opt_algebraic.cpp

@@ -866,6 +866,7 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
             one = new(mem_ctx) ir_constant(1.0, op2_components);
             break;
          default:
+            one = NULL;
             unreachable("unexpected type");
          }