BUG: initialize object array of array on resize and zeros

np.zeros(2, dtype=[('k', object, 2)]) did only initialize the first
element to zero while the rest stayed None.

    In [1]: numpy.zeros(2, dtype=[('k', object, 2)])
    Out[1]:
    array([([0, None],), ([0, None],)],
          dtype=[('k', 'O', (2,))])

This is a surprising and likely not intended behavior which is fixed
here.
The changed function PyArray_FillObjectArray is only used with None or
zero inputs from numpy, though as its part of the API it could affect
third parties but this is not very likely.

Additionally the memory after the first element was not initialized when
the object was resized.
Closes gh-4857
hm, we could possible make this saver by performing a PyObject_Copy