Fix possible crash in `actor` hash function
Fix possible crash in `actor` hash function

The crash occurred when passing a default constructed actor handle to
the hash function.

actor-framework/actor-framework,DavadDi/actor-framework,DavadDi/actor-framework,actor-framework/actor-framework,actor-framework/actor-framework,actor-framework/actor-framework,DavadDi/actor-framework,DavadDi/actor-framework