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.

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.
Merge pull request #64 from earl/fix-op-eval

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