Release buffer if function returns prematurely

[radarhere]

Resolves #4329

In PyImaging_MapBuffer, Py_Buffer view is set up, and when the image is destroyed, it is released. However, if the function returns before that, then view is not released.

The setup calls PyObject_GetBuffer, which is documented to require PyBuffer_Release - https://python.readthedocs.io/en/stable/c-api/buffer.html#c.PyObject_GetBuffer

Successful calls to PyObject_GetBuffer() must be paired with calls to PyBuffer_Release(), similar to malloc() and free(). Thus, after the consumer is done with the buffer, PyBuffer_Release() must be called exactly once.

So this PR fixes a memory leak that is reported in the issue.

[radarhere]

Regarding the lack of coverage, it is pre-existing, and I'm not convinced that these lines can be covered, practically speaking.

To trigger "buffer has negative size", PyObject_GetBuffer in PyImaging_GetBuffer would have to return a negative length.

To cause ImagingNewPrologueSubtype to fail, either we would need to run out of memory, or a very large file would be required to trip an overflow check without first tripping "buffer is not large enough".

[hugovk]

Thanks!