[bugfix] return early in websocket upgrade handler

[NyaaaWhatsUpDoc]

Fixes #1314

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

i learn so much from reading your code kimbe, this little bit is just an example of that: it hadn't occurred to me to nest the checks like this, and also didn't think of using const to hint to the compiler like this

[tsmethurst]

Looks good to me! Feel free to merge if you're happy with it :) Thanks for fixing my rather slapdash code :')