decode() should return -1 when finished

[radarhere]

I made a mistake in #6069, when I wrote that for PyDecoder, when decode() is finished, it should "return 0 for the bytes consumed." It should be a negative number instead.

This can be seen from the ImageFile code

Pillow/src/PIL/ImageFile.py

Lines 257 to 259 in 397a940

	n, err_code = decoder.decode(b)
	if n < 0:
	break

and also from Tests/test_imagefile.py

Pillow/Tests/test_imagefile.py

Lines 193 to 196 in 397a940

	class MockPyDecoder(ImageFile.PyDecoder):
	def decode(self, buffer):
	# eof
	return -1, 0

which causes the test suite to fail if the return value is changed to 0, 0 instead.