Fixes https://github.com/cython/cython/issues/4296

If there was an error in preparing the function arguments after a
memoryview had already been created, then the memoryview was not
cleaned up correctly.

(This leaves it in the slightly odd position where memoryviews
are cleaned up in the wrapper function on failure, but in the
main function on success. I kind of think it'd be better to clean
them up in the wrapper function in both cases, but I'm reluctant
to mess with a system that largely works.)