Fix ZeroDivisionError in DdsImagePlugin (#9272)

Author: hugovk

Tests/images/zero_mask_totals.dds

@@ -380,6 +380,11 @@ def test_palette() -> None:
         assert_image_equal_tofile(im, "Tests/images/transparent.gif")
 
 
+def test_zero_mask_totals() -> None:
+    with Image.open("Tests/images/zero_mask_totals.dds") as im:
+        im.load()
+
+
 def test_unsupported_header_size() -> None:
     with pytest.raises(OSError, match="Unsupported header size 0"):
         with Image.open(BytesIO(b"DDS " + b"\x00" * 4)):

Tests/test_file_dds.py

@@ -333,6 +333,7 @@ class DdsImageFile(ImageFile.ImageFile):
     format_description = "DirectDraw Surface"
 
     def _open(self) -> None:
+        assert self.fp is not None
         if not _accept(self.fp.read(4)):
             msg = "not a DDS file"
             raise SyntaxError(msg)
@@ -516,6 +517,8 @@ def decode(self, buffer: bytes | Image.SupportsArrayInterface) -> tuple[int, int
                 # Remove the zero padding, and scale it to 8 bits
                 data += o8(
                     int(((masked_value >> mask_offsets[i]) / mask_totals[i]) * 255)
+                    if mask_totals[i]
+                    else 0
                 )
         self.set_as_raw(data)
         return -1, 0