Asan reports stack overflow in validator

asan reports a stack overflow when applied to myshader.spv ([attached](https://github.com/KhronosGroup/SPIRV-Tools/files/4055216/tofile.zip)). Command is:

```
spirv-val myshader.spv
```

The shader is invalid (something that the validator correctly picks up when run without asan), but I don't think there should be a stack overflow.

Issue found while developing spirv-fuzz.

Start by running spirv-val on the attached myshader.spv reproducer from tofile.zip, both normally and under ASan. Trace the validator's handling of this invalid shader and identify why the ASan run overflows the stack. Done means the invalid input is still rejected without a stack overflow under ASan.

Written by the indexing model from the issue text.