BUG: fix np.fromiter corruption when reusing a StringDType instance (#32012) - #32117

In [2]: sd = np.dtypes.StringDType()

In [3]: a = ["a"*18, "b"*18]*10_000

In [4]: np.fromiter(iter(a), dtype=sd, count=len(a))
Out[4]: 
array(['aaaaaaaaaaaaaaaaaa', 'bbbbbbbbbbbbbbbbbb', 'aaaaaaaaaaaaaaaaaa',
       ..., 'bbbbbbbbbbbbbbbbbb', 'aaaaaaaaaaaaaaaaaa',
       'bbbbbbbbbbbbbbbbbb'], shape=(20000,), dtype=StringDType())

In [5]: np.fromiter(iter(a), dtype=sd, count=len(a))
Out[5]: Fatal Python error: Segmentation fault