Fuzzer crash e540

[dplassgit]

To repro:

bazel test -c opt //xls/fuzzer:run_crasher_test_2026-09-17_e540

Error:

Error: NOT_FOUND: Output `out`, instance #127 holds X value in Verilog simulator output.
=== Source Location Trace: === 
third_party/xls/simulation/module_testbench.cc:370
third_party/xls/simulation/module_testbench.cc:456
third_party/xls/simulation/module_simulator.cc:510
third_party/xls/simulation/module_simulator.cc:550
third_party/xls/tools/simulate_module_main.cc:208