Do not allow floodfill to extend into negative coordinates

Tests/test_imagedraw.py

@@ -559,6 +559,24 @@ class TestImageDraw(PillowTestCase):
         # Assert
         self.assert_image_equal(im, Image.open("Tests/images/imagedraw_floodfill2.png"))
 
+    def test_floodfill_not_negative(self):
+        # floodfill() is experimental
+        # Test that floodfill does not extend into negative coordinates
+
+        # Arrange
+        im = Image.new("RGB", (W, H))
+        draw = ImageDraw.Draw(im)
+        draw.line((W / 2, 0, W / 2, H / 2), fill="green")
+        draw.line((0, H / 2, W / 2, H / 2), fill="green")
+
+        # Act
+        ImageDraw.floodfill(im, (int(W / 4), int(H / 4)), ImageColor.getrgb("red"))
+
+        # Assert
+        self.assert_image_equal(
+            im, Image.open("Tests/images/imagedraw_floodfill_not_negative.png")
+        )
+
     def create_base_image_draw(
         self, size, mode=DEFAULT_MODE, background1=WHITE, background2=GRAY
     ):

src/PIL/ImageDraw.py

@@ -437,8 +437,9 @@ def floodfill(image, xy, value, border=None, thresh=0):
         new_edge = set()
         for (x, y) in edge:  # 4 adjacent method
             for (s, t) in ((x + 1, y), (x - 1, y), (x, y + 1), (x, y - 1)):
-                if (s, t) in full_edge:
-                    continue  # if already processed, skip
+                # If already processed, or if a coordinate is negative, skip
+                if (s, t) in full_edge or s < 0 or t < 0:
+                    continue
                 try:
                     p = pixel[s, t]
                 except (ValueError, IndexError):