hexl: doesn't play nicely with dynamic-completion-mode

[Chris Moore]

In GNU Emacs 22.0.93.2 (i686-pc-linux-gnu, GTK+ Version 2.8.20)
 of 2007-01-23 on trpaslik
configured using `configure  '--with-gtk' '--prefix' '/usr/local' '--with-xpm' 
'--with-jpeg' '--with-png' '--with-gif''

make a file with some content:

  $ date > /tmp/date.txt

start emacs:

  $ emacs -Q

enable dynamic completion mode:

  M-x dynamic-completion-mode RET

visit the file:

  C-x C-f /tmp/date.txt

enable hexl-mode:

  M-x hexl-mode RET

type some text containing full stops:

  hello.world.

notice that the letters insert themselves properly into the hexl
buffer, but the full stops corrupt the buffer.