floating point exception (core dump) in DCTFeatures when block_size and block_overlap are equal

To reproduce:

>>> from bob.ip.base import DCTFeatures
>>> extractor = DCTFeatures(10, (14,14), (14,14), False, False)
>>> from bob.io.base.test_utils import datafile
>>> import bob.io.image
>>> import bob.io.base
>>> path = datafile('grace_hopper.png', 'bob.io.image')
>>> img = bob.io.base.load(path)
>>> import bob.ip.color
>>> gray = bob.ip.color.rgb_to_gray(img)
>>> gray.shape
(600, 512)
>>> dct = extractor(gray)