deform_grid adds random pixels even when the displacement is an array of zeros

[EmanuelNk]

I was using the following code:

X = Image.open('im.jpg')

# resize image
X = X.resize((200, 200))

# convert to grayscale
X = numpy.array(X.convert('L'))

# define a crop region
crop = (slice(50, 150), slice(0, 100))

# generate a deformation grid of 0 
displacement = numpy.zeros((2, 3, 3))
# displacement[0, 1, 1] = 25

# deform full image
X_deformed = elasticdeform.deform_grid(X, displacement)
# compute only the cropped region
X_deformed_crop = elasticdeform.deform_grid(X, displacement, crop=crop)

# the deformation is the same
![output](https://user-images.githubusercontent.com/53420011/215512271-65317d8b-3022-403d-b570-e1c32c7ac899.svg)

numpy.testing.assert_equal(X_deformed[crop], X_deformed_crop)

even while setting the displacement to 0, the result is very noisy and there are random patches in the picture