bug#44100: 26.3; ERC: Make IBuffer custom display work again

[Olivier Certner]

Applies to 26.3, but also all more recent versions as well.

Bug trigger:
1. Load ERC (e.g., open some IRC connection).
2. Launch ibuffer (e.g., type "M-x ibuffer").
3. Switch formats three times (e.g., type "`" three times).
4. You get this error: (wrong-type-argument listp erc-notice-face) and
the buffer is only partly filled, and then cannot be updated.

Root cause:
erc-ibuffer assumes (in (define-ibuffer-column erc-modified ...) form)
that erc-modified-channels-alist is an alist whose values are proper
lists, whereas they are dotted lists.

Patch:
To be attached shortly after bug creation.

-- 
Olivier Certner