[BUG] Segmentation fault with loop over memoryview followed by an exception

[da-woods]

Describe the bug

Iterating over a memoryview in a generator expression causes a crash (on cleanup I think). pandas-dev/pandas#46029 (comment)

To Reproduce
Code to reproduce the behaviour:

# testmviewbug.pyx
def f(const double[:] qs):
    for x in qs:
        pass
    raise ValueError()

Build it with cythonize -if testmviewbug.pyx

and run it in python to give:

>>> import testmviewbug
>>> import numpy as np
>>> testmviewbug.f(np.array([1.,2.,3.0]))
Fatal Python error: Acquisition count is -1 (line 3421)
Python runtime state: initialized
Traceback (most recent call last):
  File "testmviewbug.pyx", line 4, in testmviewbug.f
    raise ValueError()
ValueError
Aborted (core dumped)

Expected behavior
No crash

Environment (please complete the following information):

• OS: [e.g. Linux, Windows, macOS]
• Python version 3.8.12
• Cython version current master

Additional context

Bisect says it started in ab1d728, which makes sense because it's a commit that'd significantly change what was generated.

[da-woods]

This only happens on the exception path. I think the issue is the line:

__PYX_XCLEAR_MEMVIEW(&__pyx_t_2, 1);

which only appears on the exception path and doesn't seem to be matched by an associated incref

[da-woods]

I do have a fix for this which I'll submit in a bit (once I've got some proper tests for it)