Check all reserved bytes in FLI header

[radarhere]

Resolves #9168

https://www.fileformat.info/format/fli/egff.htm#FLI-DMYID.3 lists three reserved sections in the FLI header, set to zero.

Pillow currently checks the first of these.

Pillow/src/PIL/FliImagePlugin.py

Lines 52 to 54 in 31eee6e

	if not (_accept(s) and s[20:22] == b"\x00\x00"):
	msg = "not an FLI/FLC file"
	raise SyntaxError(msg)

This PR checks the other two as well.

I've also update some of our test images from oss-fuzz to adhere to this new restriction.