Return -1 when finished decoding

src/PIL/BlpImagePlugin.py

@@ -306,7 +306,7 @@ class _BLPBaseDecoder(ImageFile.PyDecoder):
             self._load()
         except struct.error as e:
             raise OSError("Truncated BLP file") from e
-        return 0, 0
+        return -1, 0
 
     def _read_blp_header(self):
         self.fd.seek(4)

src/PIL/ImageFile.py

@@ -664,7 +664,7 @@ class PyDecoder(PyCodec):
 
         :param buffer: A bytes object with the data to be decoded.
         :returns: A tuple of ``(bytes consumed, errcode)``.
-            If finished with decoding return 0 for the bytes consumed.
+            If finished with decoding return -1 for the bytes consumed.
             Err codes are from :data:`.ImageFile.ERRORS`.
         """
         raise NotImplementedError()

src/PIL/MspImagePlugin.py

@@ -148,7 +148,7 @@ class MspDecoder(ImageFile.PyDecoder):
 
         self.set_as_raw(img.getvalue(), ("1", 0, 1))
 
-        return 0, 0
+        return -1, 0
 
 
 Image.register_decoder("MSP", MspDecoder)