UBSAN error in GetBackingStore()

The call to `GetBackingStore()` which was introduced in https://github.com/jeroen/V8/pull/117 seems to trigger an ubsan error.

Reproduce with `Dockerfile` like this:

```
FROM wch1/r-debug
RUN RDsan -e "install.packages('V8')"
RUN RDsan -e 'ctx=V8::v8(); ctx$get("new ArrayBuffer(8);")'

# real world example
# RUN RDsan -e "example(wasm, package='V8')"
```