Room Avatar change with invalid image will result in the wrong letter for fallback icon

[f0x52]

Description

The fallback is based on the alt-text which is '$displayname changed the avatar' instead the preview icon should reflect what Riot will actually display as the room avatar, the first letter of the room name

Steps to reproduce

• change room avatar to something Riot won't render as an image (svg, random file)

Version information

Riot develop, browser agnostic