Memoryview temps weren't being properly incremented (at the start
of the loop) or decremented (at the end of the loop). They were,
however, decremented in the exception handling case.

Long-term I think we really should unify reference counting for
memoryviews further. It's *slightly* different from PyObjects
(mainly that it happens in wrapper functions rather than the
main function) and that leads to a large chance that it just
gets missed.

Fixes https://github.com/cython/cython/issues/4662