bson_mutator.cpp declares a static BSONDomainMap called default_domains_.

It's consumed in methods like

BSONObjImpl& BSONObjImpl::WithAny(const std::string& name) {
    _inputElements.try_emplace(name, default_domains_);
    return *this;
}

If a caller every invokes BSONObjImpl::WithAny in the context of a static constructor defined in another TU, it is undefined whether default_domains_ will have constructed.

The FUZZ_TEST macro uses a static object to register tests.

Thus, we must move default_domains to a singleton, which initializes the object on first use. This will prevent the potential SIOF.