[8chan] Don't count stub (POW) image as successful download

[brachna]

Windows 10, Gallery-DL 1.35.1

After downloading a thread and getting a few normal images, this image starts to show up, gets downloaded and counted as success:

sha256: 9800cf5f26820f24f52ccdbb910564957ee17fdb863eec28afd66a65716e58dd

Guess I need to increase 'sleep' to avoid it, but there still needs to be a check against downloading it.

gallery-dl --cookies "cookies.firefox-private.txt" --proxy socks5h://localhost:9150 -i "G:/gallery-dl/_lists/8chan_list.txt"

Contents of 'cookies.firefox-private.txt':

# Netscape HTTP Cookie File

8chan.moe	FALSE	/	FALSE	0	POW_TOKEN	fef79206dc897166ccfbc8243473d964861ba1e5b4f2c17fa1992126f8cc931c
8chan.moe	FALSE	/	FALSE	0	POW_ID	cd4c26cabff4258d8b4852c46a200e08e1aaa9710d1ea441063751dfd17ac61d
8chan.moe	FALSE	/	FALSE	0	inbound	/[REDACTED]/catalog.html?pow=457465&t=17786f189dbead67e7bc3549677b5fa85d502bb98c09721a7be48595dc29c757
8chan.moe	FALSE	/	FALSE	0	TOS20250418	1
8chan.moe	FALSE	/	FALSE	0	captchaexpiration	Tue, 03 Mar 2026 23:04:47 GMT
8chan.moe	FALSE	/	FALSE	0	captchaid	697fdae153b9f342723b42b7ZyzdKXx1NE5y9cX0MHYN1YHvlhCjZ4lI7rcXlRPK9iNv8IzI2NqagVRYBVZKQTQsSTcQ5zfdEebSiN1pCqv5hQ==
8chan.moe	FALSE	/	FALSE	0	TOS20241009	1

Config:

"8chan": {
    "sleep": "0.5-1.0",
    "sleep-429": 60,
    "archive": "G:/gallery-dl/_archives/8chan_archive.sqlite3",
    "postprocessors": [
        {
            "name": "metadata"
        }
    ]
}