Unwedging doesn't work if wedged session is new, and first one created for device

[uhoreg]

If

• Alice sends an encrypted to-device message to Bob
• Bob doesn't have any existing Olm sessions with Alice
• the to-device message cannot be decrypted

then Bob will try to mark the Alice as wedged, but AFAICT, SessionManager.mark_device_as_wedged won't do anything because it tries to mark a session with Alice as wedged, but there are no existing sessions stored, so nothing can be marked as wedged.

[richvdh]

element-hq/element-meta#2389 will touch this code so that would be a good time to fix it.

[richvdh]

Annoyingly, we did element-hq/element-meta#2389 and seem to have forgotten about this.

[richvdh]

This compounds element-hq/element-meta#2356: if the sender claims a forgotten one-time key, they will keep sending messages using the broken Olm session until the recipient starts a new Olm session.