This patch concerns the handling of an NLR-raised StopIteration, raised
during a call to mp_resume() which is handling the yield from opcode.

Previously, commit 6738c1dd introduced code
to handle this case, along with a test.  It seems that it was lucky that
the test worked because the code did not correctly handle the stack pointer
(sp).

Furthermore, commit 79d996a5 improved the
way mp_resume() propagated certain exceptions: it changed raising an NLR
value to returning MP_VM_RETURN_EXCEPTION.  This change meant that the
test introduced in gen_yield_from_ducktype.py was no longer hitting the
code introduced in 6738c1dd.

The patch here does two things:

1. Fixes the handling of sp in the VM for the case that yield from is
   interrupted by a StopIteration raised via NLR.

2. Introduces a new test to check this handling of sp and re-covers the
   code in the VM.