bug#19363: 24.4.1; Notifications can make ERC unusable

[Dima Kogan]

Hi.

I just updated my Debian install from emacs24 24.3+1-5 -> 24.4+1-4, and
my ERC is no longer usable as a result.

The failure mode is that anything I type into a channel IS sent
correctly, but nothing anybody else types shows up in the buffer. So
even in an active channel it looks like I'm the only one there, since I
see nothing anybody else types. The messages do arrive to ERC, since the
notification callback sees them; they just aren't relayed to the buffer.

This is 100% reproducible if you start emacs with just the following in
your .emacs:

 (eval-after-load 'erc
   '(progn
      (require 'erc-desktop-notifications)
      (erc-notifications-enable)
      ))

Then M-x erc and connect to any server and any channel. This is
sensitive to order and/or to timing. For instance, running emacs -Q and
then evaluating

 (require 'erc-desktop-notifications)
 (erc-notifications-enable)

does not trigger the issue. I tried debugging this, but all the
indirection in the sources makes it difficult.