Fix crash in evaluation of immediate operators (op! values)

The `ET_OPERATOR` case in `Do_Next` potentially dereferences a null pointer in `word`.

For the regular case when an op! is evaluated indirectly via word lookup (as in `do [1 + 2]`), the `word` variable will is properly initialised during the lookup before entering the operator evaluation.

If, however, an op! is evaluated directly (e.g. `do reduce [1 :+ 2]`), `word` will still be initialised to `0` and the attempt to look up the name associated with the operator in VAL_WORD_SYM(word) will dereference a null pointer (which typically crashes the interpreter).

We fix this in analogue to the implementation for immediate functions, by also using the internal "-unnamed-" placeholder (`ROOT_NONAME`) for immediate operators.

This bug was found via static code analysis with the Clang Static Analyzer.

Fixes [CureCode bug 1934](http://issue.cc/r3/1934).