[bugfix] allow store smaller PNG image than 261 bytes (#2263)

[KEINOS]

Description

This PR closes #2263:

• [bug] Bad request when trying to use very small avatars/banners  #2263

This fix avoids the "unexpected EOF" error during store when the input image is smaller than 261 bytes. This is the size of the header used to detect the image type.

Checklist

• 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 made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Great stuff, thank you very much!