BUG: fix a numpy.npiter leak in npyiter_multi_index_set

[charris]

Backport of #19396.

Resolves #19375.

Moves the Py_DECREF call one line before, so the reference count will be always decresed.

Example

import sys

import numpy as np

index = 0
it = np.nditer(np.array([43124, 1234, 123432, 123432]), flags=['multi_index'])

print(sys.getrefcount(index))

for i in range(200):
    it.multi_index = (index,)

print(sys.getrefcount(index))

Before this PR

1962
2162

After this PR

1403
1403

Note: this is my first PR on this repository and my second one in my life, tell me if you have any feedback.