Fixed effect_spread bug for zero distance

Tests/test_image.py

@@ -454,6 +454,17 @@ class TestImage:
         with Image.open("Tests/images/effect_spread.png") as im3:
             assert_image_similar(im2, im3, 110)
 
+    def test_effect_spread_zero(self):
+        # Arrange
+        im = hopper()
+        distance = 0
+
+        # Act
+        im2 = im.effect_spread(distance)
+
+        # Assert
+        assert_image_equal(im, im2)
+
     def test_check_size(self):
         # Checking that the _check_size function throws value errors when we want it to
         with pytest.raises(ValueError):

src/libImaging/Effects.c

@@ -133,6 +133,9 @@ ImagingEffectSpread(Imaging imIn, int distance)
 #define SPREAD(type, image)\
     for (y = 0; y < imOut->ysize; y++) {\
         for (x = 0; x < imOut->xsize; x++) {\
+            if (distance == 0) {\
+                imOut->image[y][x] = imIn->image[y][x];\
+            } else {\
                 int xx = x + (rand() % distance) - distance/2;\
                 int yy = y + (rand() % distance) - distance/2;\
                 if (xx >= 0 && xx < imIn->xsize && yy >= 0 && yy < imIn->ysize) {\
@@ -142,6 +145,7 @@ ImagingEffectSpread(Imaging imIn, int distance)
                     imOut->image[y][x]   = imIn->image[y][x];\
                 }\
             }\
+        }\
     }
 
     if (imIn->image8) {