Commit 3f6ffe059f64b3ebc44dc0bbc63452cb8850702b implemented PEP479 but did
not catch the case fixed in this commit. Found by coverage analysis, that
the VM had uncovered code.
This commit implements PEP479 which disallows raising StopIteration inside
a generator to signal that it should be finished. Instead, the generator
should simply return when it is complete.
See https://www.python.org/dev/peps/pep-0479/ for details.