Fix unregistration crash

[sasa1977]

This PR fixes a bug which happens if a process started with name: {:via, Registry, {registry_name, process_name, value}} crashes during the startup. In this case, instead of the original error, a no function clause exception happens in the Registry module. Minimal example:

iex> Registry.start_link(keys: :unique, name: Foo)
iex> Agent.start_link(fn -> raise "foo" end, name: {:via, Registry, {Foo, :name, :value}})

** (EXIT from #PID<0.105.0>) shell process exited with reason: an exception was raised:
    ** (FunctionClauseError) no function clause matching in Registry.unregister_name/1
        (elixir 1.10.2) lib/registry.ex:264: Registry.unregister_name({Foo, :name, :value})

[josevalim]

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