Fixed reading PNG iCCP compression method

[radarhere]

Resolves #7817

Pillow/src/PIL/PngImagePlugin.py

Lines 388 to 396 in b8722f3

	# according to PNG spec, the iCCP chunk contains:
	# Profile name 1-79 bytes (character string)
	# Null separator 1 byte (null character)
	# Compression method 1 byte (0)
	# Compressed profile n bytes (zlib with deflate compression)
	i = s.find(b"\0")
	logger.debug("iCCP profile name %r", s[:i])
	logger.debug("Compression method %s", s[i])
	comp_method = s[i]

Pillow searches for b"\0", as the null separator at the end of the profile name.
Then it incorrectly uses the null separator as the compression method, when the compression method is the next byte.

The test image is a version of icc_profile_none.png, hex-edited to trigger the following error.

Pillow/src/PIL/PngImagePlugin.py

Lines 397 to 399 in b8722f3

	if comp_method != 0:
	msg = f"Unknown compression method {comp_method} in iCCP chunk"
	raise SyntaxError(msg)