Do not update GIF frame position until local image is found

[radarhere]

Resolves #6320

#6207 (reply in thread) points out that if a GIF file contains a comment block after the last image, then Pillow calculates n_frames incorrectly, thinking there is 1 more frame than there really is.

Why does this happen? n_frames seeks through the image until it hits an EOFError.

Pillow/src/PIL/GifImagePlugin.py

Lines 112 to 121 in c456aec

	def n_frames(self):
	if self._n_frames is None:
	current = self.tell()
	try:
	while True:
	self._seek(self.tell() + 1, False)
	except EOFError:
	self._n_frames = self.tell() + 1
	self.seek(current)
	return self._n_frames

At the moment, once GifImagePlugin determines that there is more data in the file, it updates the frame position.

Pillow/src/PIL/GifImagePlugin.py

Lines 182 to 186 in c456aec

	s = self.fp.read(1)
	if not s or s == b";":
	raise EOFError
	self.__frame = frame

It doesn't consider that an EOFError might still be raised if the local image is not found.

Pillow/src/PIL/GifImagePlugin.py

Lines 283 to 285 in c456aec

	if interlace is None:
	# self.__fp = None
	raise EOFError

where interlace is only set when reading the local image.

So this PR just moves self.__frame = frame after that last EOFError.

The test image was created using a modified Pillow.