21991e6c enables `__builtin_assume()` for clang and it seems
skip SEGV on rb_class_of() with unexpected value. This test expects
a [BUG] output, so this patch causes [BUG] to show [BUG] message.

https://github.com/ruby/ruby/runs/707088232?check_suite_focus=true#step:12:230
```
  1) Failure:
TestVMDump#test_darwin_invalid_access [/Users/runner/runners/2.262.1/work/ruby/ruby/src/test/ruby/test_vm_dump.rb:19]:
pid 43128 exit 0.

1. [2/2] Assertion for "stderr"
   | Expected /^\[IMPORTANT\]/ to match "".
```